If you have the money and time to prep for the GMAT now, by all means go for it. Why not? Who knows, you may end up wanting to apply to b-school sooner than you planned (i.e. 2-3 years down the road rather than 3-5, and b-schools are starting to trend younger).
As for whether you need to re-submit the scores to the schools years later when you apply -- I don't know offhand. But, even if you have to, I wouldn't use that as the reason for taking or not taking the GMAT now (it's not that much money if I recall). It really should come down to time -- whether you have the time to focus on prepping for the exam. Good luck!
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