Mannahnin
Scion of Murgen (He/Him)
Reach and occupied space (varying amounts based on weapon length, even) were covered in AD&D starting in 1978. I think BECMI dropped them for the sake of simplicity, but inadvertently left out some basic details like fighting frontage in a 10' hallway which AD&D and Moldvay B/X DID cover.The BECM/RC rules assumed a more "theater of the mind" style of play as the default, and didn't use a battle mat or grid. Things like reach, flanking, and occupied space didn't really enter the game until later. Adding them in will be no small task....good luck!
If it were me, I'd just take the numbers whole-cloth from 5th Edition, and redraw all of the maps to make combat a little more sensible. Or not; it can also be good to force the players to think carefully about cover, battlefield position, and movement.
I'm surprised to see you write that "a lot" of the old maps were in 5'=1" scale. IME almost all of the old ones (certainly all the classics from the 70s and 80s) used 10' squares, and 5' squares only became the de-facto standard once 3rd edition came around. There were hints of it earlier; Zanzer Tem's dungeon was from the 1991 basic D&D boxed set, which was the last gasp of the basic D&D line. I guess we saw some other 5' square action in the 90s with 2nd ed, but I wasn't buying a lot of modules in that period. I wonder how common it was. I guess during the AD&D 2nd ed Combat & Tactics era they were emphasizing grid-based combat more, so it would make sense if they transitioned to 5' square-scale maps in the mid 90s, even prior to 3E.I'm not seeing any mention in BECMI/RC D&D regarding how far a character can reach with a melee weapon, or how much space on the map a character occupies. A lot of old dungeons are done in 1"=5' scale, but with some tiny, two square rooms that would never properly contain a D&D combat if we assume the modern 5' area for a character in combat.
While I could allow several characters to cram onto a map square rules-wise, so many of my models are mounted on 1" bases. I'm thinking of instead upscaling these old dungeons to 1"=10'. Or is it intentional that Room #3 of Zanzer Tem's dungeon isn't the party ganking a lone Goblin, but rather an unplanned 1-on-1 brawl in close quarters?
As Marc already quoted, Moldvay B/X helpfully recommends (as I think BECMI overlooked) each character taking up 5', which is simpler than, for example, AD&D 1st edition's 3.33' (three characters abreast in a 10' corridor) baseline, which was modified based on what exact weapons characters were armed with.
Anyway, your approach of up-scaling that map to four times the size by reading the 5' squares as 10' squares can definitely work. I've done the scale flip with some Dyson Logos maps when I wanted a more open or a tighter layout.
OTOH, playing Room #3 as a tight little brawl, where the characters can't bring their superior numbers to bear, is also a valid approach. As is Theater of the Mind, with the DM just judging based on real-world distances. In reality, a few people (especially if they're missing their weapons or just have daggers, and aren't trying to swing 3' swords around) COULD squeeze into a 5' x 10' room, especially if they were jumping on and dogpiling a lone guard.