Fund wants to renegotiate percentages

Unless you really need to be trading this year (the opportunity set is 1STD+ rich), I would outright refuse the attempt to renegotiate and liquidate the book immediately to preserve the positive track record. Once you open the door to renegotiation, what's to stop your manager from doing it again or nickel/diming you repeatedly?  He's already shown himself as untrustworthy; it will only get worse from here.

Your story of "my strategy did well, but the fund won't honor my contract so I'm leaving" is going to get you tons of interviews. You didn't lose money, blow up, or have a compliance violation which are the three big red flags bdev looks at when hiring new PMs. If your current % is average for market, leaving now will probably even be a profitable career move!

Makes zero sense to treat someone this way. The way to motivate people is to offer them more resources, capital, grow their strategy. More money for the firm and everyone. Just an insulting move and seems this person really does not like you. 
As to your point about giving up what you built/enjoy this is exactly why he asked he knows the leverage they have on you but thats the MM game.

No chance in hell. Don’t let the inertia get to you. Nobody who asks to renegotiate contracts like this is worth working for. You will have zero problems finding another job. Call attorney asap as well. Good luck

Generally agree with previous commenters.  It's almost always ridiculous to ask to renegotiate when the original agreement is between two big boys (a fund and a professional).  Your new boss says your current structure is generous, but why was it set that way?  Presumably you gave up something else of value.  That something else could be a lot of things, whether we're talking about less generous pay in other scenarios or less flexibility or giving up another offer or whatever.  He can't just look at your current likely pay scenario in isolation and expect that to come off as anything other than dishonest.

The exception is the situation where he is offering something else valuable in return.  I don't know what that might be . . maybe he can structure some kind of downside risk protection or extreme pay in the upside.  Possibilities run the gamut.  But the point is, it has to be a trade that leaves you better off when looking at the range of outcomes.  It doesn't work if it's just a request to make your situation objectively worse.  Would defeat the whole purpose of the original negotiation.

Just kind of spitballing here - Everyone (MMs) are under immense pressure to reduce fees right now. They are trying to push more netting and costs down to the pod level. % payout come under fees and they likely have realized that your fee % is likely on the higher side of your var/vol allocation. the only advice I would give is you can say fuck off (hard to do) or you can try and me a little creative. Structure your deal with a hurdle rate where the % is lower (that what you have now) for the first Xmm and then escalates higher for X+. Generally a fair deal for all involved.

Simply unprofessional.  You see the same with longer non-competes that more and more firms try to establish in between.

If this is happening to you then be a proper hedgie. If they want to change your compensation then they should have to pay you for it. Be like, okay fine. Pay me for the change then. I am not giving up something for nothing. I am a PM. You know I am better than that.

If you work at a major HF, you have the brand. You should also be able to point to actual results. You should comfortably be able to get another job if they are like gtfo