I've read it. Glad things worked out for you.
So if you become level 25 with 25 perks. And at level 50, still have 25 perks but different ones... That looks pretty balancing to me. He who didn't use this mechanic, can go to 50 perks of pure class for whatever his preference was. 50 perk jack of all trades included.
I'm not good with rules: 25 max change, every change cost one. Does this rule work for above?
Well I was thinking of something a little bit different. Let′s say you are level 49 and accidentally spent 1 point in combat when you just wanted to have 50 in being a mage, then you could swap the 1 point for a magic point, going from "48 magic + 1 combat + 0 stealth" to "50 magic + 0 combat + 0 stealth" and with a certain degree of restraint you′d not be jumping everywhere around. A good system would be one that only allowed you to start changing around late, like at level 10 you would have 3 in combat, 2 in magic and 5 in stealth when you realized you wanted to be a pure warrior, then it would maybe progress like this.
10: 3 Combat - 2 Magic - 5 Stealth -- Somewhere around here he decides "I just want to be a warrior"
11: 5 Combat - 2 Magic - 4 Stealth
12: 7 Combat - 2 Magic 3 Stealth
13: 9 Combat - 2 Magic - 2 Stealth
14 - 11 Combat - 2 Magic - 1 Stealth
15 - 13 Combat - 2 Magic - 0 Stealth
16 - 15 Combat - 1 Magic - 0 Stealth
17 - 17 Combat - 0 Magic - 0 Stealth -- Goal Achieved, he is now a warrior
18 - 18 Combat - 0 Magic - 0 Stealth
19 - 19 Combat - 0 Magic - 0 Stealth
... etc
As you see he didn′t become a pure warrior until at level 17 when he started making the change as early on as level 11 and it does make you have to deal with your choices if you are not quick, someone at level 30 with 15 Combat - 2 Magic and 13 Stealth is already too late to become a pure mage if the max is 50, since taking 1 from combat and stealth in turn for 20 levels still leaves him with 5 in combat and 3 in stealth and only 42 in magic at level 50. So it′s not like it would be "WOOSH change!" or anything like that. But if he had decided to be a pure Combat player he could have achieved that around level 45, some levels before the estimated max, so it still favors you sticking to your plan.
Those thinking my story is one that kinda fits mostly since I was so young at that age do have a point, but hey I don′t have much to work with, being only 18 I have no idea if I′d have a easier time retaining my current skills by the time I became 30 if I did the same thing or not, I do know people learn and forget things faster at a younger age (namely during their childhood years) but it wouldn′t be impossible to bend the rules of reality a tiny bit for the video game Skyrim and just pretend a level 30 is able to forget 13 levels of Stealth by the time he hits level 43.
What I′m more going after is the fact that you need to practice your skills to keep them, most would grimace at my suggestion thinking "so there is still no consequence for choosing the wrong path in the beginning ?" Well what I mean is that if you make a mistake initially and get a combat skill when you mean to go pure mage, and let′s say for the sake of relevance that your characters combat skill is represented by his ability to make a stone skip on water, but he goes for the path of the mage and gets 1 more magic point when he levels, but he only got 1 because in theory he practiced his skills at skipping the stone by going to the lake at least once a day to skip stones for half an hour, but if he instead lost his 1 combat skill and got 2 magic skills, then in theory that would represent if he skipped going to the lake and spent the gained time on practicing even more magic!
I realize this becomes moot for someone who has always been pure at something since he doesn′t have skills to retain, but I bet Bethesda would be able to work out something, perhaps you could get one free perk at level 50 but only in your area of expertise if you stayed pure, it could be something like...
Dedication to X: Your dedication to X over the span of your life has made you even more proficient at it than your less dedicated peers, you gain one free perk that you can freely spend in X but not in Y or Z
How would that sound ?
And after all, if one would at level 25 start to move his skills from stealth to magic, he could as well just start spending points in magic and end up with 25 magic and 25 stealth rather than 25 magic and 0 stealth, kinda pointless if you don′t get some plus for removing a skill you have. I do have one other idea though, perhaps it would be possible to have to remove 2 skills to add to another, that way pure players would benefit from sticking to their decision and those that changed early on would not be damaged too much (someone with 1 in every skill at level 3 that wanted to go for pure thief could have 49 skills as a thief at level 50 that way rather than 1 magic, 1 fighting and 48 thief), but someone who started to change later would be more crippled (doing it with 10 in all at 30 would result in you having 40 thief skills at level 50 rather than the 49 if he′d changed at level 3, or even only 30 if he was 15 warrior and mage at 30 when he decided to be a pure thief by 50).