Really nothing is compatible between B1 and 4.1. Different engine, even maps wont work, as weapon spawn system was changed.
Liarrr!!!
The bsp files are still compatible and will load.
However, you may run into several issues (I), they all got a reason (R) but there's also a solution (S) for every single one.
I: models won't load due version number incompatibility.
R: valve didn't change much on the mdl's they only changed the version number and added jigglebones support.
S1: open the .mdl with a Hex editor, the first 5 characters should be IDST,
change the , to a 0 so you get IDST0 and save the file.
now it will load the prop.
OR
S2: simply include all the ep1 props into the bsp using PakRat.
The engine will skip the version check for every prop included in the bsp and since you'd have to add them to the map anyway, you should choose this method.
Also, there are many props which were updated since beta 1, by including props and textures into the bsp you'll make sure that the original prop and texture will be used and nothing will be overwritten.
I: Ladders are malfunctioning, certain gamemodes won't load, issues with weapon- and ammospawns.
R: Beta 1 - 1.1h used a different ladder style then alpha, beta 3 - 4 and v4.1 now. Also there have been many changes and additions to the entities, in order to support all the new gamemodes and features.
S: allthough in general i don't encourage decompiling, in this case it's the best and fastest way.
open the decompiled map with the v4.1 hammer setup.
update the entities to v4.1 standart (add info_player_mi6, info_player_janus, edit the weapon and ammo spawns and configure the weapon slot parameter, add all required entities for HL2 style ladders and make sure these entities don't touch any solids, add tokenspawn entities for gamemodes such as CtK).
Compile the map under a name which differes from the original since we'll need the original map for the next step. A fast compile without HDR will be enough.
Now open both maps with a entity editing program such as EntEd. Find all the altered and added entities in the compiled version (disabling/deleting all entities which remain unchanged will speed up this process a lot). Copy the new ones to the original map and replace the old weapon and ammo spawns with the new ones too. Save the original map and do a testrun of it in v4.1
A few more usefull hints:
- If you need to rename the map (e.g. if there's allready a new version of it in v4.1), make sure to run the buildcubemaps command to get reflections working again.
You might also need to fix the soundscape and add a levelmusic_mapname.txt to your map.
- adding, deleting, replacing entities won't change the CRC of a map, including content such as props, textures or even the levelmusic script will change CRC and filesize.
- you might need to fix up paths for some txt files such as levelmusic or soundscape when including them into the bsp, since they need relative paths but somehow (bug?) will be added with absolute paths.
