This question isn't easy to answer, because ironically our evidence for Greek armour in this period is not as good as it is for earlier periods.

The unfortunate fact is that the Greek habit of dedicating armour at sanctuaries like Olympia more or less stops around 500 BC, and burials with armour were never very common on the Greek mainland to begin with. As a result, there are no more huge hoards of helmets like the collection of Corinthian helmets from the Archaic period found at Olympia. Most of the surviving material is scattered single pieces, many of them not from the Greek mainland.

For armour of the Classical period, our most important source is iconography (images from vases, coins and sculpture), of which there is much more than for earlier periods. But they present a problem of interpretation. While we get a lot of depictions of Greeks in battle gear, we cannot be sure that these depictions are true to life. The way warriors are presented in art is heavily determined by social attitudes and political agendas as well as artistic conventions. The argument that Corinthian helmets went out of style is largely based on their disappearance from the archaeological record paired with their rarity in Classical iconography - but perhaps they didn't really disappear, but artists became more interested in depicting poor hoplites than rich ones. Art of the Archaic period mostly catered to the wealthy elite, but that of the Classical period reflected a wider, more democratic audience. Alternatively, perhaps artists transferred the convention of depicting cavalry without armour, even though literary sources tell us they would be heavily armoured. It's hard to be sure.

Whichever way we interpret surviving images, though, it's clear that Corinthian helmets were no longer anywhere close to the default. Many other types were available, and the lighter ones would have been more common just because they were cheaper, if for no other reason (and most scholars argue that there were in fact good practical reasons as well, such as increased sensory range and freedom of movement). It is possible that some hoplites would still be wearing Corinthian helmets; there were certainly no rules against it, and no surviving text expresses any preference. But from the near-absence of Corinthian helmets in art of the Classical period we can conclude that they would have been at best a small minority that may have been regarded as old-fashioned (and possibly impractical and inferior).

As for body armour, the problem is the same. Many warriors in this period are depicted without armour, but we do not know if this is accurate, or a nod to the "lowest common denominator" hoplite, or an artistic convention. Some generic hoplites, like those on the Nereid monument from Lykia, are still depicted wearing linen cuirasses, although the same monument also features hoplites with just tunics on. Some literary evidence has been taken to suggest that hoplites c. 400 BC would not be wearing much armour, such as the implied availability of only a handful of cuirasses among the 10,000 hoplite mercenaries of Kyros the Younger. On the other hand, we know that the bronze muscle cuirass only fully developed in this period, which suggests that it was still worn by wealthy hoplites (although more commonly by cavalry).

In short, while we cannot be absolutely sure, it does seem likely that the typical hoplite of the early 4th century BC would be wearing an open-faced helmet type and little to no armour. This was of course no impairment to his fighting ability. The hoplite's main defensive gear had always been his large round shield. As long as he had that, he easily outclassed any more lightly equipped infantry, and would consider himself reasonably well-protected against both missiles and close combat weaponry.