Hi everyone , can you put a 3 inch bore decat on say a 2.5 inch bore and so on ?

Not without a lot of exhaust paste...

 

I have seen Ant on Wheeler Dealers fabricating such a downstep tube, but he is brilliant at metalwork, and had a very specific (expensive looking) machine to do it LOL

Oh, just found this on Ebay...(Cheap too!)

I bought this cheapo decat from ebay 2 years ago and has been fine (although they've put the price up by a £15 since I bought it). I wouldn't bother with a branded de-cat as long as it's stainless.

You'll also find decats that are straight (notice how the one i linked has a small kink in the middle of it). The straight ones make your exhaust tips go a little wonky as it pulls the exhaust over (the original decat is also kinked).

If you haven't had the car remapped then you will get an error code for exhaust emissions. Fit one of these elbows where the o2 sensor goes and it should help stop the light coming on all the time.

To actually answer your question, the cat is flanged either side so is pretty easy to remove (apart from the o2 sensor if it's been in for 15 years, have fun with that). Both flanges for the 2.5" stock system and 3" decat are the same dimensions in terms of bolt holes, so yes it will fit fine with whatever system you have, and plan on using at a later date.