how many holes and do you have access from above?
You can get a holesaw arbour with a longer thread. This allows you to put the 75mm holesaw on and then fit the 60mm holesaw inside it.
The 60mm holesaw then becomes a 60mm pilot bit and allows you to drill your 75mm hole.
I normally template hole and cut with dry-wall saw but......
You could make a template, like this:
If you can cut a 60mm piece of ply or plasterbord (ideally the thickness or approx of your ceiling) with 60mm hole cutter, so you have 60mm disc, that fits snugly in to exiting holes.
Then cut a bigger piece of timber/platerboard, this can be any shape, so it overlaps the 60mm disc and the intend 70mm hole. Align the two together and fix/screw up together. Pilot a central hole through both these pieces, so it takes the holesaw guide/pilot
Then take trip in to loft, place the template in to the holes 60mm hole, 60mm side down, so snug in existing hole and resting by means of larger piece, put a weight on top to restrict movement of template, but don't cover guide hole, then you can drill from underneath.
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