You might be able to convert it using the finaldemo to 2.0 converter found on the website first and fix it from there. It's under historic downloads at the bottom of the page if you care to try.
First thing you're going to have to do at the very least (assuming this worked in the first place) is maybe remap any and all upside down springs to be gravflipped regular springs. Springs also work under gravity now so any springs floating in midair will require a specially SOC'd spring to replace it or some other means. Springs have had some minor updates in terms of strength as well so this will need to be accounted for as well for sections like GFZ2's spring segment.
Second thing is a few object types have been moved around in terms of number so you're going to have to make sure the objects are the correct types.
Third, and this is kinda the big one, any SOC related custom objects have to be mostly reworked due to the fact that we changed from a number based system to a word based system and certain objects may try and overwrite others in the vanilla game. Any custom objects found in these levels will most likely have to be reworked or converted using a converter which i'm not even sure works for 1.09.4.
Lastly, if you care about visuals at all, you're going to have to use the srb2.srb iWAD from finaldemo loaded in as a seperate thing in SLADE for converting any and all textures and flats to not only the current palette, but also to a format that the current version can use, otherwise your floors will be corrupted looking and your textures will all be redwall.