Reuse existing insulation attic

[EVEN]

Hello
I want to convert my attic into living rooms.

the roof is currently insulated with rock wool placed on a rafter (air gap about 7 cm) and there is flocculated type rock wool 10 cm thick between the joists supporting the ceiling.

About the roof several companies have advised me to keep the existing one and to install 100mm insulation over it.
I understood that in this case I must remove or tear (12 slots per m2) the existing vapor barrier.
should I also remove the existing scoth between each insulation panel?
I plan to put a hydroregulating vapor barrier under the BA 13.

Concerning the ground I must provide a joist reinforcement and installation of osb on resilient tape. I plan to reuse loose rock wool for sound insulation in particular. should I put a vapor barrier as an underlay? for my part I would say no
part of the current ceiling of my lower rooms is made up of a wooden lath pointed at the current joists, the rest is made of bricks suspended from the joists.

What do you think? thank you