My 1999 WRX is well overdue for a timing belt replacement- Only done 75,000kms but 20 years old!
I am going to do it myself and am concerned about camshafts getting out of sync so am considering cutting the existing belt along its length , removing and then sliding the new belt into position over the various sprockets and rollers.
Once located , the remaining old belt can be cut away and the new slid over into its final position.
Has anybody tried this with success ?

personally no i wouldn't its always a good idea to change the water pump out too due to the ease of access, you really don't to get this wrong and use a good quality timing belt not some ebay special.

Yes, it is time.
I've been told to replace at 7 or 8 years even if the odo reading is very low.
Your method sounds interesting, but what happens if you have sliced the old belt and cannot get the new belt on? Do you have a plan B?

You could do a search on this forum, I recall some discussion on DIY belt replacement.
As Rslib72 says- this is not a job to get wrong.
and at 20 years, replacing the water pump would be sensible.

I wouldn't cut the old belt. I suspect it would be hard to do, they are tough as, and I needed my old one to help undo the sprockets. I used monkey pliers to hold the sprocket still against the crank sprocket - I have a pic someone posted ages ago if you need it.

Get good tools and a breaker bar (I seriously considered buying an air gun). Worst part of the job by far was getting the engine off the tranny which required a butchers cleaver and a mallet and I owe a debt of gratitude for this video (I hope you can imagine how desperate I was before trying this:
https://www.youtube.com/watch?v=sSsN...ature=youtu.be )

I did it on my own, engine out, because I needed to do the clutch too. I reckon it would be near impossible with engine in. Having a helper to keep everything aligned would really...help.

The actual alignment on my own was a real faff, but it was very obvious once it's all right and I'm told you can be one tooth off and not contact anything on a full rotation. To be more than one tooth off you'd need to be a numpty. That said, my sprockets and alignment marks were stock and easy to see. I suggest taking the old belt off after marking it against every contact point and then refitting it to get an idea of what is involved before you start with your new belt which will only have one timing mark.

It's not a job for the faint hearted, but not hard (thanks to the interweb) if you have a good tool set and are mechanically minded. I had to get the engine out again the following weekend as I shagged the new rear oil seal 1st time around and had an oil leak which doesn't help the new clutch. I used an aftermarket seal there initially and went OEM 2nd time around. No leaks there, but the friggin rocker box gaskets are another matter.