the other piece of note is that the preview "upgrade" to the final version actually keeps your win 10 as an insider beta tester FOREVER. so you will keep getting updates and such for your win 10 but you will still be getting them before "standard" users and providing feedback to MS as a permanent beta tester.
some people don't like this idea and won't want to use this as an avenue to get a free copy of win 10. as for your win 7/8 key, from what i have read it looks like you will have 1 year to upgrade to win 10 but can still go back to win 7/8 if you chose and lose your freebie win 10 key. it does sound like you will have to chose to lose your win 7/8 key to use the free win 10 upgrade they are offering.
but to answer your question...