Will the QR Codes from my free account be deleted if I sign up for a paid plan?

No. your free account is independent from your paid plan and you can keep both accounts separately.

QR Codes and Landing pages from a free account do not expire.

Take into account that they have a limitation of 100 scans/day that you cannot exceed or the account will get blocked.

You can import QR Codes from your free account to a paid plan.

However, it is not possible to transfer QR Codes from a paid account to a free account.