So the big problem in answering this question is that we have very little idea whatsoever as to how pre-Christian religion in these regions functioned on a day-to-day basis. Even our knowledge of Norse mythology is based on sources written by Christians, sometimes centuries after conversion. Our main literary source for the practices of Norse Paganism is Adam of Bremen, who wrote his Gesta Hammaburgensis ecclesiae pontificum in the mid-11th century, however it soon becomes clear that Adam either didn't really care about recording the truth, or simply couldn't be bothered to do any research: large tracts of his descriptions of Scandinavia and the Scandinavians are lifted almost verbatim from Pliny's descriptions of Africa in his Historia Naturalis. Alongside his accounts of human sacrifice and giant golden idols at Uppsala, he also recounts how the Vikings go to breed with the Amazon women of Russia, who then give birth to dog-headed cynocephali, among other, more fantastical creatures. While the conversion of Scandinavia was indeed a slower and more gradual process than elsewhere in Europe, therefore, we have no way of knowing if there were attempts to reform Norse Paganism (if even there was an organised system of religion or priestly caste to reform it) because even if it had reformed, the Christian contemporaries who witnessed it would likely not have deemed it worthy of recording, if they had noticed it at all.

It is worth noting as well that the conversion of Scandinavia was largely spearheaded by the Scandinavian kings themselves. I wrote a little bit here about the useful advantages which were available to an ambitious early medieval monarch upon conversion to Christianity, and why it was so popular. Any Norse revivalism, therefore, would have been largely a tool of dissent, and faced royal opposition.

The conversion of the Slavs, particularly the Wends, is a very different concept. We actually have a relatively detailed picture of the everyday religious ceremonies and shrines of the Wends, as recorded by Thietmar of Merseburg in his Chronicon in the late tenth century. Unlike the Scandinavians, however, the Wends were largely forcibly converted to Christianity via German conquest. There certainly was opposition to this, but in the form of mass armed uprisings. Thietmar records numerous accounts of churches attacked, one in what appears to be some form of localised zombie apocalypse which is far more likely to have been a small Wendish uprising. In the aftermath of Otto II's disastrous defeat at Stilo left the bulk of Ottonian military power in disarray on the other side of the Alps, the eastern territories of the Holy Roman Empire were struck by a series of highly destructive Pagan Wendish and Slavic uprisings which Thietmar mourned set the mission of Christianity 'back by several decades' in those territories.