I would always take the pan out and tile underneath, then replace the pan, but then I am a plumber....
As for just bedding the pan on silicone... NOOOOOOOOOOOOOOOOOO don't do it! The pan MUST be screwed to the floor - that is why it has screw holes in it. I've had to sort out too many pans that have moved and are leaking poo all over the floor because the silicone under them has given way. Its not a pleasant job.
If you are worried about moving the loo and the pipework not being long enough then get yourself a flexi connector - you'll probably need to take the end off the original pipework to get it to fit on, but so long as there is a handy isolation valve its not a problem. A push fit flexi hose will make your life even easier if you are not that confident with the pipework side of things. Failing that call a plumber :wink_smile: