taking away casting/conjuring/preparing versatility in addition to chargen Karma in exchange for power points versatility is a perfectly viable balance mechanic if you ask me.
A mystic that today get magic A get 10 spells and no power points.
He can spend 25 of his chargen karma to get 5 power points.
He now get both a lot of casting/conjuring/preparing versatility in addition to power points versatility
He suffer because he don't have karma to flesh out his character with positive qualities, focus, skills, attributes etc.
With my proposed house rule he would get 5 spells and 5 power points for 10 karma.
He then spend the remaining 15 of his chargen karma to get 3 more spells.
He now still get both quite a lot of casting/conjuring/preparing versatility in addition to power points versatility
(but he only get 8 spells and 5 power points instead of 10 spells and 5 power points)
He still suffer because he don't have karma to flesh out his character with positive qualities, focus, skills, attributes etc.
Getting both casting/conjuring/preparing versatility and full power points will be more expansive with my proposed rule.
By core you can make a mystic into an identical magician except that mystic does not have astral perception or projection and have the option to get power points during initiation later.
With my proposed house rule you can still do that.
By core you cannot make a mystic into a physical adept with just a few spells on the side.
You can't trade casting/conjuring/preparing versatility for power points.
With my proposed house rule you can now do that. Sort of.
A physical Adept with priority B will get 6 power points and 25 spare karma.
A mystic at priority B will get 4 magic and 7 free spells.
To get 6 magic he would either need to get 2 more special attribute points from metatype or get a A magic priority
1) If he get 2 more special attribute points from metatype to reach 6 power points:
This will cost him 12 of his 25 chargen karma and all but 1 of his free spells.
The "cost" for getting 1 free spell and the option to spend karma on more spells during gameplay is 2 special attribute points and half of his chargen karma.
He also does not get power points (not free nor with karma) when he raise magic during gameplay
2) If he get a higher resonance or magic priority to reach 6 power points:
This will cost him 12 of his 25 chargen karma and all but 4 of his 10 free spells.
The "cost" for getting 4 free spell and the option to spend karma on more spells during gameplay is a higher magic priority and half of his chargen karma.
He also does not get power points (not free nor with karma) when he raise magic during gameplay
A mystic that goes magic A can now choose anything between 10 free spells, 25 starting karma and 0 power points down to 4 free spells, 13 starting karma and 6 power points. A mystic that get power points will, compared to a magician, get less chargen karma, less casting/conjuring/preparing versatility, no astral projection and have to buy astral perception. That is the cost for having power points out of chargen and the option to trade metamagic for powerpoints during gameplay.
Perfectly balanced imo.