Hi Przemysław,
I somehow doubt that you will get a definite answer for the second one. That's also true for my reply. I have only a little experience in this area and known related to WEBCON BPS. In the end you will probably have to test it.
Nevertheless here are my thoughts about this topic:
The first one is definitely not a problem, if you would switch from Mixed to windows i wouldn't be so sure. In some version the bps_user was added and I'm not sure how it would behave in Windows only mode.
Regarding the second one I don't see a problem, if the new collation is less strict than the existing one. For example going from case sensitive to case insensitive shouldn't cause a problem, while the other would. But than the collations are part of the Database and you wanted to change it of the SQL Server.
There could be one problem if the Collation of the BPS databases differ from the tempdb. I'm not sure whether this is explicitly used by WEBCON BPS.
Best regards,
Daniel