Most adhesives have priming agents already in them these days so you don't need to use them in all cases. I think the BS says about tiling on tiles.... ensure they are fully fixed, never tile directly over each tile copying the grout joints if the tile sizes are the same, key them with your Tile Scrive to allow the adhesive to bond to the backing of the tile and not just the surface, ensure the substrate is sound, so if it's plaster you'll probably find it can't take too much more weight, if it's plasterboard you should be ok to go over them.
I've always been lead to believe that you don't need to PVA much these days. If you use it when you don't need to, you're only creating an extra layer that you're hoping doesn't fail in the future. Providing you've double checked that the adhesive doesn't require a PVA layer to be applied firstly, don't do it.
Realisticly, if you take them off you will probably find that they are not fixed as well as they could have been, especially if they were done some time ago when adhesives lacked what they have today. The thing is, if you take them off, you'll probably find you're going to need to path up a fair bit of your wall, or dot and dab plasterboard over a couple of the walls, usually the external ones which are in most cases render and plaster, assuming it's not a modern newbuild.
You're fine using the Type 2 Class AA (oldskool term I think it's a D1 these days?) to go over tiles directly providing they are scored well enough. But really you're providing a guarantee to the customer that the existing tiles are fixed well, which you don't really know fully, even after trying the tap method on them and checking the substrate is sound etc.
Like Darren, I'd try to pull them off as best you can at least on the external walls.
