Is it possible to center align text in the Speech Bubble annotation in Preview.app

Probably this is not possible with Preview, but there is another (free) PDF reader that allows you to do this (at least your first three points, don't know about converting into another annotation kind): Skim.

After you've opened your PDF in Skim, you can choose Convert Notes... in the File menu. Then you have all your notes from Goodreader in the drawer panel on the right.

It does work in Lion, the way you tried it.

In Snow-Leopard you have to select the text itself to change the font(size) by first clicking/selecting the text-field to get the selection rectangle (for re-sizing or moving - as you described) and then click a second time on your text to select all the characters. Now your text should be selected (visible by the highlighting) and you will be able to change the font-size in the Text Properties field.

Edit 2012-01-04:

I could reproduce the problem described and also found a simple solution. If you open your pdf and re-arrange pages or rotate them and then try to change the formating of existing text fields it (sometimes) fails.

The solution/fix is to click the window (menu-)bar of the "Fonts" window, then click on your text field again and finally chose the new font variant → and it will be applied :D