Hi,
If you have a subscription version of Office, you can install it on your new PC anytime by signing in at account.microsoft.com/services. Subscriptions let you use Office on multiple devices, so you are free to set it up again on your new computer. If you already done it try to install Office again using the correct volume license setup. This clears conflicts and fixes the activation problem. If the license does not match and Excel keeps asking for activation. The files are fine, but you need to remove any old Office leftover and install only your Microsoft 365 Home. After that, the files should open normal again without any activation message.
I hope this helps!