Original plans had customers entitled to a 24 hour support response time when their Software Assurance spend was more than $250,000 USD - as of next year though, you'll need a Support Contract.
That means either a Unified Support arrangement across your enterprise, or the uptake of pay-per-incident support (also available as a 5-pack option that expire within a 12 month period), which doesn't seem a particularly attractive proposition to smaller organisations. If you do have a Unified Support Contract you can transfer any current Software Assurance 24x7 Problem Resolution Support incidents before February 1, 2023.
And the costs - well Unified Support is customised based on the actual spend, where pay-per-incident is published at Professional Support For Single Incident – $499 USD for 1 user. Professional Support For Five Pack Annual – $1,999 USD for 1 User.
So you might need to revisit those 2023 forecasts just to make sure you've covered any additional costs you might incur - or maybe challenge Microsoft to their commitment that "Our goal is to ensure every customer gets comparable support services at a comparable price, despite the benefit retirement." !