I believe that the 338-08 would require a new bolt face. I am not really familiar with either round, but if the 338-06 is a 30-06, necked up for the 338 bullet then I do not see why a BAR in 30-06 could not be just rebarrelled. The bolt face is already designed for the 30-06 case, so no modifications would be required within the receiver. My guess is you could do this for under $300 depending on the quality of the barrel you purchased. Hopefully there is a smith on the board that can give you a better answer.