For the topic of the DCE:
Yes while some labs
may have been holding off and using 4033's(and possibly later compiles) code for stability, doesn't mean its core code would undergo little change. For instance, as far as we know, lab04 could've take 4033 and gutted its mil engine for 4039's. This would mean it would not be compatible with the older one in 4033, forbidding the idea of copying over files from 4040. Not only that, but its shell may have been cleared up with more 4039ness as it
appears to be at the level of functionality with 4039, with just 4033's browseui further forbidding copying.
We just don't know because we don't have this build with most of this being based on inferences on info available and discussion with Melcher. Best bet is to contact grabberslasher, he may have tested it and know much more.
On the topic of themes:
4042.main still uses 4033's theming engine or one based on 4039's. Same with 4040.lab04. So therefore its guts may not work with builds that have the v7 'modern' UI. Each longhorn build is unique in multitudes of ways from another.
4042.lab06 may not be newer than 4042.main. So, not all builds post 4042.lab06 may contain slate

.
While 4039's plex can be used under 4042+, I still can't find a way to fix the shrunk title text:

Oddly enough, 4039's Luna theme can be used with a properly enlarged title, but its text color isn't on par. Indicating some sort of config issue with plex and its title. Changing anything under advanced appearance settings in desk.cpl does nothing.
And for
splitwirez, this is the progress I've made with 4039izing 4042:
If I use 4039's sidebar:

The tiles don't work right.
But here's my 'in progress' of swapping 4042's resources:

Still haven't changed start button or taskbar. May just stick with 4039's sidebar and get the tiles working.
I have logonui changed as well.
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Back to 5063, this is quite an amazing build. It has most of 5112's features, yet much earlier. This may have been the result of several labs having worked on Omega-13 before others, resulting in much more completeness early on.
This brings up the question: when the labs were bringing over Longhorn features to Omega-13, what did the UI look like? While it would be largely incomplete, it may look rather nice as they went through thinking of what to use.