I'm with Babaganoosh -- there was no "behind-the-scenes reason" for the change from UESPA to Star Fleet (sic), because it wasn't that well thought-out. They were just making stuff up as they went and seeing what stuck. The two episodes that mentioned UESPA were both by D. C. Fontana, so it must've just been a term she liked. But it didn't ultimately catch on.
ENT established that Starfleet existed before the Federation did, but in the late fourth season we saw a Starfleet insignia reading "STARFLEET COMMAND -- UNITED EARTH SPACE PROBE AGENCY":
http://memory-alpha.org/en/wiki/Image:Starfleet_UESPA.jpg
According to Memory Alpha, the Friendship One probe (from VGR) launched in 2067 had the UESPA logo on it, although that'd be odd because there was no United Earth yet. Also, set decorations from NX-01 in ENT and the Enterprise-B in GEN both mention UESPA personnel involved in the construction of the ships. This suggests that UESPA existed as an organization for a very long time and was either affiliated with or the same organization as Starfleet.
How about this: Maybe "Starfleet" is shorthand for "the United Earth Space Probe Agency's Starfleet."
ENT established that Starfleet existed before the Federation did, but in the late fourth season we saw a Starfleet insignia reading "STARFLEET COMMAND -- UNITED EARTH SPACE PROBE AGENCY":
http://memory-alpha.org/en/wiki/Image:Starfleet_UESPA.jpg
According to Memory Alpha, the Friendship One probe (from VGR) launched in 2067 had the UESPA logo on it, although that'd be odd because there was no United Earth yet. Also, set decorations from NX-01 in ENT and the Enterprise-B in GEN both mention UESPA personnel involved in the construction of the ships. This suggests that UESPA existed as an organization for a very long time and was either affiliated with or the same organization as Starfleet.
How about this: Maybe "Starfleet" is shorthand for "the United Earth Space Probe Agency's Starfleet."