The original CZ 527 front sight was a dovetail set up held in place by a spring loaded plunger. My only suggestion would be to make sure the plunger was fully depressed and lightly tap the sight forward with a non marring hammer. I have swapped the front sights before but never have they stuck in place. You might try some penetrating oil before tapping. For some reason like rust, gummed oil, a to tight original fit the sights hanging up. If you can get it started it will probably finish easier.