As for the lifetime keys remember this: Lifetime licenses were discontinued as of March 2014 and can only be used to activate Malwarebytes Premium on a Windows device. I am lucky to be Grandfathered and have 2 Lifetime keys for Malwarebytes.
So, if it now working and You don't want to reactivate it again with inserting key to the same computer again (it may increase the key inserting number), I would recommend to bacup the activation data. If I remember correctly, it's stored in in %ProgramData% folder, there should be folder Malwarebytes or something similar. Check it.
If you uninstall malwarebytes on that PC, the key comes free again. Or if you before uninstall press the button; Deactivate License. The third option is to deactivate the key in Malwarebytes My Account online. So there is really no need for back-up or anything. Hence this will not work anyway as your Machine ID has changed with the re-installation. Malwarebytes keeps track of this on their License server.