Trying to open a saved level, Into a different mode

With one level i made it wont open saying "please select a map to load for editing" but the other does open fine and they literally have the same selected resources.

It was previously "GZDoom2 Doom Format" when i made it and now I want to make it "GZDOOM2 UDMF" for access to the script icon in the top left

You can't open a map in one format as a different format. You have to convert it to the new format, or copy the map to a new map in the correct format.

Someone pointed me to a file convertor called WinZWadConv.exe which I used to convert an old vanilla Doom II map into UDMF format. Might have been this site's admin, I don't remember.

Edit: It was DoomKid, here's the info he sent me:-

1 - Download https://doomshack.org/~doomkid/Utilities/WinZWadConv.zip, select the map you want to convert for "in", and set a new wad name for "out". (If the program refuses to run, right click and set the "compatibility mode" to either Windows 7 or Windows XP)

2 - Using Ultimate Doom Builder / GZDB, open the newly converted wad in "GZDoom: Doom2 (Hexen format)".

3 - Go to Edit --> Map Options, and change the Game Configuration to "GZDoom: Doom2 (UDMF format)". A warning will come up, click Yes.

Now, save your map and close. When you try to re-open it in Doom Builder, it should be in GZDoom: Doom2 (UDMF format)!

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You may need to fiddle in Slade after, as I had an eroneous file in the wad that had to be deleted.