It appears that when I rename any of the files/tabs through wiindows
explorer, the sketch won't run anymore. It works fine in previous version
147. But version 155 gives me a "Syntax error on token(s), missplaced
construct(s)." error message

I got around it by making a new sketch in 155 and recreating all the tabs
with new names and pasting the code into it. But I think this is a bug of
some sorts. I'm sure you guys are working on a way to rename tabs. But
the manual process of changing them seems to causing some problem in 155.

You should never rename a file of a sketch that's currently open. There's
no way for Processing to magically know when you've done that inside the
Windows Explorer.

Rename tabs using the on-screen menu, described under Help > Environment:
http://processing.org/reference/environment/index.html#Tabs