There were plenty of right wing nationalisms in the 19th century, though I know less about them. These were generally pro-monarchist nationalism, though perhaps you're right they're mainly post-1848 (things like German nationalism). There were also a lot of relatively left wing nationalisms of this period that coexisted with right wing nationalism in this period--Armenian nationalism, for example, was fairly pro-socialist while the earliest forms of Kurdish nationalism were quite religious (Turkish nationalism occupies a fairly ambiguous position in a left-right classification, being quite revolutionary in social relations, but also having several conservative and authoritarian characteristics as well).

There have also been plenty of left wing nationalisms in the 20th century. All the Marxist wars for "national liberation" for example. Contemporary Kurdish nationalism is quite left wing, especially in Turkey and Syria but even in Iraq and Iran (in the latter too it's often more tied to traditional tribal power). Arab nationalism, while it was a potent force mid-century, was quite left wing, in both its Nasserist and Baathist forms. Zionism (aka Jewish nationalism) was predominantly left wing for most of the 20th century, and only after the 1967 did more right wing and religious nationalism start becoming more prominent. The Palestinian nationalism of the 1960's was left wing (often aligned with Arab nationalist trends) and only saw a rise of religiously mobilized groups in the 1980's (Hamas, Islamic Jihad).

So there are quite a few cases of recent left wing nationalisms (and this is not even getting into "indigenous" movements and anti-colonial movements). In general, one could say that left wing nationalisms seek to alter the state (either by breaking away from an existing state or changing the nature of social relations) and right wing nationalisms seek to maintain the status quo (either by maintaining social solidarity or resisting giving power to minority groups or resisting a Marxist-style change in social relations). There are also effects of global waves: Nader Sohrabi has a few great articles about how global waves of constitutionalism changed the Middle East, Japan, and Russia in the early 19th century. We often think of waves of democratization (Huntington's three wave model, with the famous third wave being post-1974). There were likewise waves of anti-colonialism and Marxist national liberation. Groups associated with this wave were left wing and often nationalist. Groups opposed to this wave were right wing and often nationalist. Since the 80's, arguably, there has been a wave of increased religion in the public sphere (I wish I could give you a simple causative explanation as to why, but it likely has its origins in anti-communism and was bolstered by the collapse of the Soviet Union). This wave was sometimes left wing (liberation theology in Latin America) but more frequently associated with the right (in the Anglosphere, Middle East, India, Eastern Europe). Sometimes it's quite nationalist, sometimes (as in the case with some Islamist mobilization) it's anti-nationalist. The relationships between all these moving parts strikes me as quite complex and difficult to articulate in a parsimonious model.

I dispute that nationalism has any favor of one over the other. Japan is/was as right-wing as you get, and it has always been fiercely nationalist. Same is probably true of China, and they've gone from one extreme (an imperial throne) to the other (socialism) in the opposite way while maintaining the nationalistic focus. I would say they're probably even more so now than they've been in quite a while. France is fiercely nationalistic, and always have been. Flag-waving expansionist Napoleonic France was left-wing? I don't think so.

I would dispute the claim in the question that nationalism was a concept associated with the left wing in the 18th and early 19th centuries. Unless of course the OP may be referring to a specific country instead of in general. If this is not the case my objections to the premise of this question is the following:

Firstly because modern political concepts like left and right wing is precariously difficult to apply to earlier historical periods, especially before the French Revolution where the term was conceived and for a long time solely applied to people with regards to their opinions on the various events unfolding there.

Secondly because nationalism historically as well is a disputed concept, with accepted scholarly theories ranging from it having existing at least since the Middle Ages in Europe, to it originating in the early 19th century. The term "patriotism" was fondly used throughout the 18th century, but in that setting it cannot always be directly translated into nationalism, and it was most certainly used by people of every political observance, at that time it was already a Grundbegriff, a "basic concept" as laid out by conceptual historian Reinhart Koselleck, and was in that regard devoid of content on its own, but used by everyone for very different purposes.

Thirdly because one huge contemporary discussion in the period specified by the OP was the discussion regarding cosmopolitanism or universalism. Many Enlightenment thinkers, especially those that Jonathan Israel would term "radical", which in the 18th century pre-Revolution period would be among the thinkers considered most progressive, was in favour of cosmpolitanism, but there were adherents on all sides of the spectrum.

As such I would find it difficult to answer how it changed from being associated with the left wing when that claim is in itself questionable.

Nationalism changed from a civic sense of nationalism, simply regarding ones country and its inhabitants, to an ethnic sense of nationalism. This change occurred in North America when immigration began to rapidly increase, meaning the population was slowly becoming filled with non-americans and thus nationalists had to distinguish themselves.

Source: Eric Foner - Story of American Freedom

wait? Is there even a clear defenition of what left and right means politically?