You are getting some great feedback here!
My deeply devalued 2 cents:
What is the reason the school has such a time in grade requirement? Maybe they have a school/style-specific answer.
1-I don't put much stock in belt ranks per-se--they tend to differ wildly from school to school--esp between different organizations even in the same style. So no.
2-Couldn't care less--they might progress faster or slower depending on how much they need to "un-learn" in order to progress at their "new" style---not really much much point to additional training if all you are going to do is the same stuff you already are doing.
At that point it's less a question of doing it "effectively" its a question of learning how to make a NEW set of skills/techniques just a "effective" as your "old" set.
Take an extreme example---say someone comes in that is good enough to beat the teacher--do they get an automatic high ranking in TKD? Probably not, no-one is questioningtheir fighting ability but rank in a given art means a lot more than just the ability to fight (IMO :)) I presume that high rank implies having a pretty good grasp of the history of the art and the ability to teach the specific's of that art. Mike Tyson and any number of UFC fighters are major league tough---but I'm not sure that they could teach TKD.
3-I'd go back to question 2--why exactly is someone training?
4-Don't think that "time served" is set in stone. But depending on the reasons you have such requirements I'd make it pretty hard to get around them.
What might be interesting is if you could get a look at the requirements of the seniors in your group---I got a look at the tests once of group I belonged to and found that the requirements---ALL the requirments, INCLUDING time in grade had been SIGNIFICANTLY added too over the years. 20 years back the same guys telling me I needed "X" amount of time in grade and a 2 day long test were testing for the same rank with VASTLY less time in grade AND requirements than I was being asked.
NOT bad guys in any way, shape, or form----just there is a strong tendency to "add" material as time goes on.
Something to consider for "time in grade" requiremtents is there is a big difference between really knowing something and just being able to spit it back out on a test. Pretty easy to get a learners permit and start driving---but how long does it take to REALLY be a good and safe driver???
There is something to be said for taking the time to really work with material---there are people out there (non-MA people BTW) that would argue that to really know what your doing---to really understand a skill and do it correct you need (roughly) to practice it for what works out to around 10 years.
On the other hand if people are sticking folks with time in grade requirements just to get more money out of them?.
Then that bites!