Form many years shareware, crippleware, or breakware were all the rage. To truly have a 30 60 90 use life you must get creative. Use a 'Matryoshka' file. Take one of the files in the install, the eula file for example;

END-USER LICENSE AGREEMENT FOR THIS SOFTWARE
Important - read carefully:

This End-User License Agreement ("EULA") is a legal agreement
between you (either an individual or a single entity) and the
mentioned author of this software for the software product
identified above, which includes computer software and may
include associated media, printed materials, and "online"
or electronic documentation ("SOFTWARE PRODUCT").
By installing, copying, or otherwise using the SOFTWARE PRODUCT,
you agree to be bound by the terms of this EULA. If you do not
agree to the terms of this EULA, do not install or use the
SOFTWARE PRODUCT.
Inside, take several lines and count spaces. Extra spaces are typos and one or two here or there no biggie. Do a check before you launch your app, if the file is missing or the space count is wrong, Dialog an error/warning and close. If the count is correct, use the file mod date to get your count to REGISTER date. When they register/pay send a small patch to overwrite the 'Matryoshka' file with a corrected one. No extra files, no registry edits, just a correction for typos. You can leave the scan code in place if you want, as all future counts will be correct. This is old data and should be treated as such.