If we have thousands of plots, can we avoid destroying them and using up SSDs?

Will I need to replot to use the official pooling protocol?
Yes. Anyone who wants to join a pool will need to create new K32 or above portable plots. This new plot format allows you to switch between pools and self-pooling with a cooldown of 30 minutes (rules set by pool) between each switch. Each switch between pools will require a transaction with a smart contract on the blockchain. Our recommendation is to slowly replace your existing plots with portable plots one by one, so you still have a chance to win XCH while you convert to all portable plots.

Can I farm with both OG (original) plots and portable plots?
Yes. The farmer will support both OG plots and portable plots on one machine.

Can we keep our current plots to continue to farm in solo and in same time create the new plots (portable or the new k32) for only participate to a pool ? (a part of your space to farm in solo with “old” way to make plots and another part of your space for pooling (or self-pooling) The goal is to avoid to destroy the old plots, above all if you had already 500,1000 plots ! or more.

Yes, that’s exactly what the FAQ says

Our recommendation is to slowly replace your existing plots with portable plots one by one, so you still have a chance to win XCH while you convert to all portable plots.

Replace your existing plots by re-plotting.