I obtained a takeoff barrel for a factory marlin 1895 Cltd, it isa 24" 1/2 octagon, 1/2round in 45-70.
I had an early 1895 that I initially wanted to change barrels on but elected to buy a new 1895 for this purpose.
Is it a big deal to switch barrels or is this a gunsmith job.
Any gunsmiths that specialize in marlin s

Define "big deal".

Being the barrel was fit to another action it could be a big deal to change the barrel. At the worse you would have to recut the shoulder of the barrel so the sights would index correctly and then you would have to make all the adjustments to the tennon, including the extractor cut, and re-setting the headspace. Then you would probably have to adjust the forearm hanger and barrel bands. Or, you could be extremely lucky and have it fit just right. Either way, you won't know until you or a gunsmith try it.