1) Nothing to stop you asking for IFR but that means you need to be separated from other IFR traffic which may actually make the transit more difficult. VFR only needs traffic info on IFR.
2) If you're VFR and given a VFR transit the clearance phraseology should be...
Cleared from (place) to (place) VFR via (routeing) not above (level), maintain VMC while in the (name) control zone.
Note the bold. You can't change to IFR at your own whim, it needs to be done by agreement with ATC and well before any possibility of going IMC as there are separation implications in doing so, both from other aeroplanes and from the ground.
If you want IFR, ask for IFR. If you want VFR, ask for VFR. Changing from VFR to IFR once inside the CAS is probably not the best plan and should be avoided if at all possible.