The way the device works is there is a little roller coaster track inside (ok think a kiddie roller coaster at the county fair). The roller on the vacuum pump rides around the track and bobs up and down. If you happen to have the engine stopped in a location that the roller is on top of one of the "hills", it will be very difficult to remove because of the stress being placed on the bolts due to the springs being compressed on the roller arm.

So if you start backing off the bolts (go evenly in a criss cross pattern) and you feel pressure building then stop. Get a 27 mm socket on a breaker and rotate the engine (clockwise only!) until you feel the tension release. Now you are in a low spot. Then take the bolts the rest of the way out and swap it over.

Your new pump's roller will no doubt be sitting at a different angle, so when you go to put the new one on, you might need to again rotate the engine to get the new roller in a low spot on your gear track.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech