Other than "I will not comply" what say you to the purchase and installation of magazine extensions on mags that are already greater than 15 capacity?

Say I want to add a +2 baseplate to a Glock 17, 17 round mag that I owned prior to July 1. Could I order a +2 and install it?

What about extensions that are themselves greater than 15 rounds as in a +18 nordic extension for a PMag.

My reading of 1224 says to me that I can't make a new configuration that holds more than 15 but is silent on modifying one that was already greater than 15. Is that wishful thinking on my part?