Product review (5 stars): Replacement cartridge made easy
This is the mfr recommended replacement cartridge since Samsung discontinued the 20A.  No problems at all putting it in.  I didn't shut off the water to the fridge, and I could feel some resistance when I turned the filter to lock it in.  If I had shut the water off then there would probably be less resistance, but still I was easily able to install it.

The pricing made no sense at all.  The single pack was $30; the double pack was $90; the triple pack was $94.  So just order 2 or 3 singles and scratch your head as to why they're priced this way.