I have a bathroom with very sound tile floor. The owner builder framed in a mortar bed 50 years ago and it is solid. I want to enlarge the bath into an area that has 1x6 t&g subfloor. Can I bring this area up flush to existing tile with hardibacker or something and then tile over the whole area,both the old tile and the hardi? In another spot I have demoed out a shower pan and would have a similar build up to do with underlayment for vinyl sheet goods. If this works at all how to skim the seam between new underlayment and old tile floor. Or glue underlayment to tile and then cover? Thanks for any advice.