-what will happen "to the address format "if i import SegWit private key [P2SH or Bech32] into un-SegWit wallet "not support SegWit"?
That's easy to test: create a new segwit address from segwitaddress.org (note: I do not know if this site can be trusted for real use, do your own research!), and enter the private key for Wallet Details on bitaddress.org.
Result:Segwit Address: 34mUxBdWrXzjuCy3iwHypQm1bZhJ7oCnJZ
Segwit Private Key: L1F1mh5m7rzYEsdioVHJRMZcrwss1AHwhCd3owDUnD33CAw7JiXa (
don't use this one!)
Bitaddress.org:
Bitcoin Address Compressed: 1CqC1x4ZkwH1HTDAe7UpsEu6c545udvBPr
Can sign message using that private key "if new wallet support signing message"?
You can sign a message from 1CqC1x4ZkwH1HTDAe7UpsEu6c545udvBPr, but that doesn't prove anything about your SegWit address.