If it's an "early" L3901, yes absolutely change the fluid in the transmission, as well as the suction filter. Kubota had an issue with contamination and a very few HST filters that would die (yours is a DT, so you won't have that filter). But I've also seen a couple 3301/3901 suction filters leak too, for whatever reason. Can't remember the serial number range, seems like it was in the 53xxx range where the cut off was. Before that, yes replace the fluid at 50 hrs. After, just filter.
It may be wise to call your dealer armed with your serial number and ask them about it; there was a bulletin for it.