If this is preseason will rank be reset when season open

Yes it will be. However, climbing in the preseason isn’t completely useless. It raises your MMR which means you’ll be reset from that higher MMR when the actual season starts.

high mmr really makes a big difference for climbing

i was able to skip silver 4 and silver 2 after placements this season. it made the trip to gold extremely quick and easy.

if you're winning games in preseason it can do nothing but help you

though losing games can hurt for the same reason ;)

if you want to keep the same rank you have now, your best bet is probably not to play too much until the new season.