I've got some stuff that I made using Allegro 4 which I now want to convert to Allegro 5. .. It's a bit of a daunting task to go through everything looking for what needs to be changed, and then changing it all and hoping that there are no mistakes... So I would prefer to do it just bit by bit.

So, my question is this: is it ok to use two version of Allegro at once? can I initialise both of them and use them side by side without them interfering with one another? .. For example I might using allegro 4 for drawing bitmaps and allegro 5 for timing and events.

(actually, it's particularly difficult for me to completely drop Allegro 4 on the project that I'm thinking of, because Allegro 5 doesn't have a GUI addon.)

I don't think using the two alongside each other is a good idea. It might work, I'm not sure. How far into the project are you? If it's early alpha, just switch to A5 and write your own GUI code (or use an add-on); if it's almost done, then keep A4 and save A5 for the next one.

99% sure they will interfere with each other to the point of nothing working.

It'll take you about one afternoon to port Allegro 4's GUI to Allegro 5. The reason is that most of the GUI code is fairly self-contained and doesn't actually depend very much on the rest of Allegro. You'll probably be spending most time on re-doing the widgets, but if you're like everyone else you probably already have custom widgets anyway.
Alternatively, people have worked on making a GUI addon for A5, you might be able to pick up one of those.

Ok, so the general advice seems to be that using both Allegros at the same time is a bad idea. I suppose I'll just sit on it for awhile until I build up enough motivation to convert the whole thing in one sitting. As for porting the A4 GUI stuff, I'll consider it, but I probably won't actually do it...

Thanks for the advice.

Speaking of A5 GUIs, it looks like Trent Gamblin has done one: http://www.allegro.cc/forums/thread/604516

Not being a user of A5, I can't really comment on the quality. But you might find it useful, Karadoc