Upgrading a Trezor wallet to Multi-share Backup leaves the original single-share backup valid. The new recovery threshold doesn't apply to that old copy.
Under Trezor's existing rules, an eligible 20-word wallet can gain a backup made of several shares, with a chosen minimum needed for recovery. The upgrade creates another backup of the same wallet, with the same accounts and addresses. It doesn't replace the original recovery route.
That leaves a decision about the old backup. Keeping it means continuing to secure a copy that can restore the wallet without the new shares. Trezor says to verify that the new Multi-share Backup works and gives access to the funds before destroying the original.
If the original backup may already have leaked, adding shares doesn't neutralize that exposure. Trezor explicitly warns that a compromised original still puts the funds at risk after the upgrade.
Source: https://trezor.io/guides/backups-recovery/advanced-wallets/upgrading-to-multi-share-backup
Under Trezor's existing rules, an eligible 20-word wallet can gain a backup made of several shares, with a chosen minimum needed for recovery. The upgrade creates another backup of the same wallet, with the same accounts and addresses. It doesn't replace the original recovery route.
That leaves a decision about the old backup. Keeping it means continuing to secure a copy that can restore the wallet without the new shares. Trezor says to verify that the new Multi-share Backup works and gives access to the funds before destroying the original.
If the original backup may already have leaked, adding shares doesn't neutralize that exposure. Trezor explicitly warns that a compromised original still puts the funds at risk after the upgrade.
Source: https://trezor.io/guides/backups-recovery/advanced-wallets/upgrading-to-multi-share-backup