The reason they made the powerlevel reset when you change class, is to prevent using one class to gain power, then switch to a class that has a transform powerlevel lower than the on you're using, and then transform extremely early with him. I don't mind the wait till you are killed part, but keeping your KI. No, sorry, just wouldn't work.
Yea, every1 would just choose buu, eat like 4-5 candy, and then switch to vegeta and be able to transform instantly, that's not exactly fun. Imo, there's no point in waiting to get killed first before you can switch, either way you're dying so why not make it sooner than later?
it is possible to keep ki if u put ur mind to it and come up with a solution that could work. Consider different ways of keeping your ki depending on different switches between classes. For example chaging class from a low trans class to a high trans class would not increase ur pl, just keep it normal. Transforming from a high trans class to a low trans class is dependable on keeping you ki based on a percentage. I.E. if your power level is 2.8 million and u need 3 to trans, and ur new class needs 1.75 mil to trans, then ur new power level will be 1.63, 93% of power related to transformation power level. That way u can switch between classes and trans only if u've reach a significantly high power level with a low trans class or with a high. Transforming with a low class would limit your class changes and high classes give u more options. Seems logical does it not?