I think that Hogdon developed the "H" series of powders to be a little slower burning, thus allowing a larger powder charge (ie, a fuller case which is usually considered to be the most accurate).
I've read somewhere that one can safely switch the two AT THE MINIMUM CHARGE listed for either. Work up from there, keeping in mind that IMR powders are slightly faster burning. However, advice that you get from the internet should be used at your own discression, so do your own research in your load manuals to decide whether to use one verses the other.
Its NEVER a good idea to try using one powders data for another powder , with a couple of exceptions ( New H110 & Win296 = same powder , per Hodgdons ) Basic loading rules state to always start low and work up , even when changing lots of the same powder .
stimpylu32