If you are keen to remove the old board material by planing/machining and you have the skill level and equipment to do this, its a valid way of doing this repair. Andrew, in his post, has sketched out the basics well.
My choice would be to jig the guitar and machine of the old material with an overhead router as it puts less stress on the guitar and the finish while the material is being removed, however a god jig setup and a plane should do the trick fine. I would pre-stress the neck to provide a little bit of built in relief to allow for any tendency to back bow when the new board is glued on. You will need a good strong caul for gluing the neck on.
I would personally prefret the board and do most of the finishing tasks on the neck before gluing it on but, if you are not familiar with how to do this (you best have a crenelated caul) prefretting can be a difficut option. But fretting after the glueing process should be conducted to minimise any pre-tension the fret tangs place into the neck (and subsequent backbow problems for single action truss rods).
If you are not confidence around fingerboard glue ups I would opt for epoxy rather than my usual go to glue, Titebond Original, to minimise any neck reaction to the glue.
One tip that I use from time to time to avoid compromising the finish at the fingerboard edge is to not take the removal all the way to the edge of the neck - allow 1/32 of the old ebony finish to remain as a buffer. The new fingerboard can be finished to the old edge and carefully blended at that juncture thereby minimising the impact on the finish which is probably a bit thin or shaky at that point anyway. Dont lacquer the new fingerboard edge - use a fine paper schedule to polish the edge to the old lacquer and then treat with a good finish oil which will blend very well in most cases.
The added benefit of leaving some of the old finish is that the truss rod and its disposition do not become an issue (remember to neutralise the truss rod tension before look at how the neck is to be jigged).
That's a bit of what we know.
Guild BHM1 was a short run series authorized by May but not exact replicas of his home made guitar, actually traditional mahogany setnecks with Dimarzio pickups and Khaller tremolos and toplocks. Very good quality, as any 80s Guild guitar, but not "authentic". They made some replicas later in '93 with the tremolo more or less close to the original design, but still solid mahogany with many conventional details. Exact replicas are made by RS Custom in Nashville and they are funny: blockboard body, oak inserts, mahogany veneer, dyed oak fretboard and repro tremolos with motorcycle springs :)
The guy who owns the Guild in question is a Queen fan and a collector, a good friend of mine, so I'm able to lay my hands on various RS replicas, Guilds, Burns, even a home made one is present.
I may be waving my ignorance again, particularly since I know very little about Electrics but...
Reading over the original post, it seems that the original finger board may have been shaved to compensate for drift in the neck angle so I'm thinking that restoring a fingerboard with the original dimensions would create another problem unless the neck was also "reset". How do you 'reset" a through neck guitar?
Yes, the neck was shaved partly to compensate for the angle, but it seems like the fretboard at the body had been thicker from the start, so when I make the new one as thick as it should be, the string height at the bridge will be ok. The frets will be the tallest anyway. This Khaller Pro tremolo has some, I'd say, unpleasant things about it: very shallow angle at the saddle, height screw gets in the way, no way to lower the baseplate except routing the body. On the RS model it's even more: zero fret and a very shallow headstock angle, short scale and usually light string gouge used. They all tend to buzz and go out of tune! Mr May designed the guitar for himself, it's not a Strat or Les Paul at all :)
It's not a through neck (neither is the original RS design though it doesn't help the unfortunate who has to reset it) but I really hope I won't have to reset this neck, just need to measure and calculate for the minimal string height at the bridge :)))
I appreciate the answers to my "off topic" question. It was my mistake to think it was a through neck but I've wondered about it before so I asked.
Well, the iron & spatule method worked great, fretboard sides cleaned of the finish first. The finish (and the base coat or sealer found under it) tends to peel a bit, leaving visible "pockets" but easily cured with thin CA. So far so good.