For a Melee player trying PM for the first time, you may feel slightly awkward. Others adjust fine. Just ask for a handwarmer before your set, and try figure out if anything feels weird. Marth, along with the other Melee top tiers, are as close to a 1:1 transfer in terms of inputs - all buffs/nerfs that would change the timing of anything were avoided. Falco, for example, is basically unchanged afaik other than his Laser damage and Dair spike duration getting nerfed. Shouldn't affect the timing of anything, but Falco mains often complain that he feels different at first. "More slippery." This is likely due to one of a few things.
Character models are slightly larger in Brawl, and the camera is a tad more zoomed out. Because of this, along with the differing Brawl animations, characters may seem to move further/shorter than usual, and this can mess with people's spacing without them realizing it. Falco usually suffer the most from this, I haven't heard Marths note that as much. On the animations - Brawl Marth's sword swings differ slightly in shape from Melee's. The hitboxes are the same, but the slightly different sword swing visual may leave you feeling you shouldn't have missed one or two whiffed U-tilts/F-smashes.
Brawl physics are pretty different, and because of that, PMDT wasn't able to make PM physics engine perfectly 1:1 with Melee's physics. One of the major differences is that your character is considered airborne for one frame extra, and is not hindered by a Melee mechanic that is sometimes referred to as the "jump gravity quirk." Quirk causes you to slow your vertical momentum if a rising aerial is performed. Combined, both of these give characters teensy bit of more airtime in PM (not visually noticeable; it's like 1/60 of a second extra). Marth especially benefits from this, as he can now perform a SH Fair --> Bair with enough time for the Bair hitbox to come out, which is impossible in Melee.
Marth's F-throw is buffed. No DI on PM F-throw gives the same distance as DI inwards on Melee F-throw, allowing you to chain grab most character weights at low % unless they DI outwards.
TL;DR - Marth has been buffed overall in PM, both from universal mechanics and character-specific changes. For this reason, many Marths don't mind the transition from Melee --> PM, but the reverse can feel a bit awkward. So try go for Melee bracket and then PM bracket, but if that isn't an option, just ask for a handwarmer before any Melee sets to make sure everything is comfortable.
EDIT: More in context of actual question in OP (I was just giving a hypothetical example where you had to play both on the same day), no. You don't have to worry about PM feeling or being bad for Marth. Just get to know all your matchups carefully, and keep up some Marth practice at home on the usual daily basis/every other day/whatever to make sure you don't get too used to PM Marth's minor buffs over time.