Those 10-round magazines are designed to be nearly impossible to convert back to 15 rounds.
During the high-cap ban magazine manufacturers had to make the 10-round limit nearly foolproof while still fitting pistols made to accept higher round count mags. To meet the law, the mag bodies were made shorter and false solid bottoms added, depth limiting grooves were embossed into the body that would break if you attempted to pound them out and other design features were used that could not be defeated at any reasonable expense.
No one simply stuck a spacer into a high-cap mag that you could remove. If you want a high cap magazine, you have to buy them in that form.