I know there are a few that got the B to C+ mod in the 1980's. Very few. So yes it has been done.
A mod in 2004 may have been the effects loop mod. Some say this leans the IIB more towards the C+ lead tone.
But as far as a flat out IIB to IIC+ change, they ran out of parts to do that years ago. So if the mod was done in 2004,I would have to say "NO"
No it really can't w/o changing the whole layout. In the beginiing there were a few (like 20) that were sent back and had the guts replaced to make them c+. These came with a letter explaining that the majority of the guts were changed. They can mod the IIB to be more like a C+ (I bought one of these) THey move the reverb to the preamp area (I think) and put a 3rd master vol on the back near the slave out.
Mine sounds great, but I have not A/B'd it with My C+. I have a post from about a month ago about this. Do you have the auction number, I'd like to see it?