Template changer does not change fontsize in Activity and Groupjive

Yeah my main worries was with the database tables. I use a multisite arrangement and any changes in db can be troublesome so keeping the original database tables as backup is important.

A suggestion: could we have a button in the backend (plugin or whatever) where we can basically "launch" the conversion of old into new? Use case: I have a main (master) site which runs the PHP, all dependent (slave) sites share the same PHP as the main site (use of symlinks...) BUT each website has a different db. I am fearing that if I install CB 3.x on the main site, then all the other sites would go down (produce errors) as I may have to manually copy (I fear) all the new db tables into the slave sites. However if we have a "Migrate now" button, then we could work out the kinks in each site on its own instead of 100+ sites suddenly going down.
Secondly: I have nearly all the current CB plugins. Would it be possible to have (together with the "Migrate now" button) a check being done by "CB Migration plugin" if all the corresponding joomla extensions/plugins have been installed, including a list (eg "Plugin XYZ has not been installed yet. Click this link to download from Joomla Extension Directory and install?").
Thirdly: You mention about new Activity Streams being done differently. I can see the potential for quite some confusion if you have to update both at the same time (I personally fear that I will struggle at first). Maybe include an option in the new activity joomla plugin which says "Legacy mode" or "New mode" (eg look at how they have done the routing in Joomla when they went from 3.4 to 3.5).

A suggestion: could we have a button in the backend (plugin or whatever) where we can basically "launch" the conversion of old into new? Use case: I have a main (master) site which runs the PHP, all dependent (slave) sites share the same PHP as the main site (use of symlinks...) BUT each website has a different db. I am fearing that if I install CB 3.x on the main site, then all the other sites would go down (produce errors) as I may have to manually copy (I fear) all the new db tables into the slave sites. However if we have a "Migrate now" button, then we could work out the kinks in each site on its own instead of 100+ sites suddenly going down.

Migration will likely be entirely automated on install of respective plugins. So for example if all you install is CB 3.x it'll only migrate CB itself. We haven't even started CB 3.x though so that might change.

Secondly: I have nearly all the current CB plugins. Would it be possible to have (together with the "Migrate now" button) a check being done by "CB Migration plugin" if all the corresponding joomla extensions/plugins have been installed, including a list (eg "Plugin XYZ has not been installed yet. Click this link to download from Joomla Extension Directory and install?").

Core CB will not be aware of optional extensions. CB won't have plugin management at all anymore in CB 3.x. You'll have to do that check manually yourself. Plugins will handle their own migrations upon install of their corresponding extensions.

Thirdly: You mention about new Activity Streams being done differently. I can see the potential for quite some confusion if you have to update both at the same time (I personally fear that I will struggle at first). Maybe include an option in the new activity joomla plugin which says "Legacy mode" or "New mode" (eg look at how they have done the routing in Joomla when they went from 3.4 to 3.5).

Activity streams will work largely the same. They'll just be standalone in CB 3.x. We'll be adding backwards compatibility where it makes most sense. For example we'll try to route old plugin URLs to the individual extensions (we'll just hardcode in a routing map for that).

It's possible a separate migration tool will be provided to allow us to avoid loading down CB with a bunch of migration code. I can't really say for sure yet, but the migration tool ideally would then cover all your concerns.