You can easily fuck yourself if you class change too much in 6 and 7. You will be lacking in levels, meaning you repeatedly get wrecked by status effects, and you will end up with a huge excess of skill points (especially in 7 when skill points are category-locked).
Very fake news. You change the classes early in game at very low levels. This way in the end you can be less than a level beyond what you would be without hyper class changing. There are some classes/races combos that allows to do it even on level 2 and they don't need any additional points at all to do it! You can go from one class to another and back again almost indefinitely. Many others class/race combos require only very few points to change the same way. Basically, you make use of the fact that early levels cost a very small % of exp of higher levels. Enemy "generators" make it even easier. If you change the class a dozen times or more you still won't be behind on higher level significantly. If it worked like you think it did, there wouldn't be any issue with Wizardry 7 levelling to begin with.
I rarely say that but in this case - to everyone who wants to play W7 - don't use this "feature". Change classes but set yourself a self-imposed limit of how many changes is allowed before the beginning of the game and stick to it. I suggest 2, maximum 3 changes. That's my advice, take it or leave it.
Wizardry 8 levelling system had serious problems too. Namely it was (almost) useless. The authors understood the problem with Wizardry 7 but what they did as a solution? They addressed the issue... by butchering the system almost completely. Class changing wasn't so easily abused anymore. It was useless instead. Except very few class changes (and even there it was minor) they simply don't make sense, or have minimal impact mechanically. I suppose it is still usable by larpers. "I want a Bard/Gadgeteer! It's cool" kind of thing.