So in such a game, where the players are invested in their characters as is....I agree that just doing a wholesale change over is not the right way. Instead, you introduce things slowly and organically.The thing is, the group I’m running for would be specifically hamstrung by Level Up’s changes. It includes an Expertise as double proficiency oriented multiclass heavy skill based character who depends on double proficiency bonus, a Polearm Master+Sentinel Barbarian/Fighter defender, an abjurerer
Wizard/Fighter 1 specializing in counterspell and dispelling enemy buffs, a monk/cleric depending on Polearm master + stunning strike for personal defense, and a ranged hand crossbow pure fighter who would be least impacted since crossbow expert and sharpshooter weren’t badly impacted on Level Up.
I’ve used the monstrous menagerie which is fine, but going over some of the rules options in core went from interest to a “nope, never” quickly when I floated the idea, for reasons I’ve already noted - so apparently my group has a different mindset on how they view compatibility. The changed magic items really got to them, particularly the ones that are designed to create resources and don’t anymore, like the decanter, but the rule changes were enough that it’s a different game.
I’ve pitched a one-off short game where we just “try it” but that spun the conversation to, “if we want to play 5e we should play 5e and if we want to play something else heres my recommendation” which covered everything from 13th Age to Pathfinder 2e to The Dark Eye.
They were pretty psyched for an add-on that gives new classes and rules but not the Level Up system that changes build concepts they rely on. The monsters are fine, but the player rules aren’t exciting then and I get why, hence passing on the “notes” that these are a big deal to players who rely on those elements for their builds.
Examples:
1) A magic bastard sword that allows you to deflect attacks once a round (aka the parrying property)
2) A spell scroll for a "special version of the spell" that the party can learn (a rare spell, or at least the A5e version of it)
3) A monster that does a special kind of mental attack that leaves a mark that requires a long time to heal (aka strife)
4) A few overland encounters that require some skill checks (aka an exploration challenge).
5) Give the player inspiration, and let them know they also have the ability to spend it on X if they wish (aka a destiny power....one you feel is suitable to the character).
6) Design your encounters using the updated A5e math (without telling the players).
7) Have the TWF in the group learn from a wise sage in advanced combat, and is now able to do two attacks with their off hand.
8) The next NPC that tags along with the group....have them provide a follower type bonus.
9) Give the players a stronghold (could be a little one), maybe a reward for one of their quests. Try out the stronghold benefits that way.
This weans them into it, lets them try things out (without truly knowing they are trying it out). And then when you run your next game, maybe you ask them about some of the things you introduced and see if they like it. If they do, well then maybe you can bring in A5e more fully. If not, then you can continue to cherry pick from the system and let them play core.... as that's a key aspect of compatibility. You pick from the system what you like, and keep the rest core....and the game should work absolutely fine.