Change default posts per page and Google Search

[Scandal]

Hello all! :)

Recently I changed on vbulletin settings the max posts per page. That was 50 per page and I turned it to 20.
As you know, Google is getting the web sitecontent as guest, so now all previous /showthread.php?t=X&page=Y pages, are showing different posts.

Is there something I could done to prevent bad google response? For example, duplicate content penalty?
Or to leave it as it as / do nothing?

Finally I want to notice that today I submitted my forum's sitemap xml (via vBSEO Sitemap addon).

Thanks in advance!
Scandal