For better of for worse, that is not at all how Medicaid funding works. There's not a defined budget of money to spend. State's determine, within federal guidelines, who is qualified and what providers will be compensated for treating them. Then payment for that treatment in total, however much it may be, is split between the state and the federal government at defined ratios (Federal Medical Assistance Ratio) that already vary by state or territory and by group being covered. So if a group, in this case adults covered under the ACA Medicaid expansion, is covered at 90% by the federal government, that means that 90% of the cost is covered by the federal government and 10% by the states. Dropping it to 80% means it is now an 80:20 split. It is not unusual to have that vary by state, the normal ratios already are calculated relative to state incomes, states more capable of carrying a higher percentage already do.
But the important thing here is that Medicaid levels of care and enrollment aren't changed by this. What the law says is that states participating in Medicaid must keep their Medicaid plan within federal guidelines, and so long as they do, the federal government will bear minimum 50% of the cost of the program. If these states say "ok, our assistance ratio dropped, so we need to cut our Medicaid program", they can't actually really do that. Their plan has to stay within the federal guidelines, they can't undercut those guidelines. And if the same people are eligible and covered for the same things, its going to cost the same in total, just a higher percentage is going to be funded by the state portion rather than the federal. The total entitlement is unchanged. When people talk about cuts to Medicaid in a meaningful way, they are typically talking about reducing the entitlement, not just shifting the purse between different parts of government.