no this isn't a hud developer dating thread fun as that might be
assuming your HUD is 100% up to date just before MYM was launched, here's what you need to do.
note: this guide is not friendly to those that don't understand hud development. don't try what you're about to see unless you're confident. wait for your hud to be updated.
here's a full list of diffs for the MYM update (edit: heres a better one), proceed to update with those changes as you normally would.
after all that you're going to need to adapt your hud to the new versioning system, which is going to make custom huds fifty times better for end users. for now, you're going to just need to add this file to the root directory of your hud (for example, for an already-installed yayahud, it would be under tf/custom/yayahud/).
for those who don't want to download a file (hurr viruses) just make a blank file in notepad called info.vdf with these contents:
"customhud"
{
"ui_version" "1"
}
edit the file and replace "customhud" with the name of your root folder (for example, "yayahud").
you're going to need to update this file whenever valve pushes a UI revision before your users will be able to use their custom huds with the new version of TF2. i assume that valve will be conveying these version increments going forward, but that's just my guess.
i tested all of this successfully when updating yayahud, so these steps should work for everyone else too. if something doesn't work for you though feel free to drop a note here, and if you figure anything else out i'd be happy to add it to the op.