OK, I'm confused. PL2 has 3 folders, one for each season (aren't there four of those in Europe? :)) ). Inside each is a folder 3dpatch. 3d patch already exists. Do I need to move the original 3dpatch and replace each manually when the season changes? So far my attempts have caused a crash as soon as I hit "fly mission".

The seasons are broken up due to size most likely. In RB3D, Spring also doubles for Summer unless you specifically make a Summer Vol file. You did download the ones with vol files and not individual tiles, right. The tiles, having all of them, can slow the game down. The main thing about individual tiles is you can then modify parts of your terrain. There are terrains that modders have made which separate Spring and Summer. the one thing is the tiles in Fall and Winter have different names, while Spring and Summer Tiles share the same bmp name.

How would I know? One has tons of what appears to be photos. I used the link on Flybert's site (I believe).

originally there were only 3 3dpatch folder .vol files
3dpatch.vol containing clouds, fire / explosion effects and spring/summer terrains
3dfall.vol containing fall terrains only
3dwinter.vol containing winter terrains only
We discovered 3dspring.vol was a valid file name for the spring / summer terrains, and that the game will read 3dspring.vol before 3dpatch.vol for those tile names for spring / summer dates
we also discovered making sub folders under 3dpatch folder with the names 3dwinter, 3dfall (or 3dautumn) and 3dspring would be read by the game before the same named .vol files
so, the easiest way to just change terrains, is to leave the stock 3dpatch.vol in place for the stock clouds and fire/smoke/explosion effects for Glide, and to use a sub-folder or .vol named 3dspring to change the spring/summer terrain
if you repack 3dpatch.vol with the modified terrains, you don't need 3dspring.vol, however you always need a 3dpatch.vol because it contains the files used by all seasons such as fire/smoke/explosion effects and the clouds
in theory, you could put different clouds in 3dwinter.vol, 3dfall.vol and 3dspring.vol and those textures would work seasonally (change with the season)
the seasons were not broken up due to size, they are broken up due to palette changes and to have a full set of textures painted differently for each season
Red Baron 2 used only one full set of terrain textures for all seasons and had only 3 changes of appearance .. 3 *seasons* .. though it's possible a date based palette change makes a subtle change from Spring to Summer appearance
all custom terrains made thus far, use only about 1/3 of the full set names in each, 3dwinter, 3dfall, and 3dspring
in the stock game, where the full set of terrain tile names is used in 3dpatch, 3dfall and 3dwinter, there are actually NINE changes in terrain appearance as time progresses through the seasons .. or at least there should be ..
in other words .. there are 9 differently painted tiles for each terrain tile *slot* in stock RB3D for Glide, 3 names for each separated by 3 .vol files

Wow, I never knew that.
EDIT: I just tried it and it doesn't work. Can't name the folder 3dfall.vol unless I wish to over write the actual vol file. By just naming the folder 3dFall, all I saw was the regular terrain tiles for fall, not the one I modified (generic1.bmp, all one color with the letters "FAL" filling the image). If the bml is in the 3dPatch folder, it shows, but will not show when it is in any of the subfolders. the folders are named: 3dpatch, 3dfall, 3dwinter, 3dsummer, and 3dspring. Now, 3dpatch is Spring, but since you mentioned 3dspring in your post, wanted to make sure that was actually needed for some reason. But in all cases, the moddified terrain tiles in the sub folders did not show.