On Twitter, Bethesda's Pete Hines was asked whether old Skyrim mods will be compatible with Skyrim Special Edition. "Basically, yes," he replied (as spotted by PC Gamer), as Thomas The Tank Engine and Macho Man dragons the world over breathed a sigh of relief.
Meanwhile, well-known modders on well-known projects like Skywind — a years-in-development mod that aims to bring The Elder Scrolls III: Morrowind into Skyrim's engine — have weighed-in optimistically. The mod's landscaper, MicahGhost, pointed to one likely (though not confirmed) upgrade: a 64-bit engine. "A 64 bit engine is great news for us," he wrote. "It gives us more power and better stability to make Skywind even better."
He added, however, that the process won't be entirely seamless. "The only thing that will need porting/updating is Skyrim Script Extender. SKSE is a special tool that gives creators more freedom with the engine, so an upgraded engine would require a new SKSE update. I do not believe it will take long at all, because the same team has already done a script extender for Fallout 4, which uses a 64-bit engine most likely almost identical to the new engine Skyrim Special Edition most likely uses."
Like Fallout 4, Skyrim Special Edition will allow mods on consoles. Unfortunately, anything that uses Skyrim Script Extender will probably be off-limits, because script extender mods directly alter games' underlying code. That means mods as elaborate as Skywind might not be able to make the jump.
Of course, in the particular case of Skywind, the mod actually needs to come out before we start worrying about console ports. Someday. Sooooooomeday.