This is from the FFXIV wiki on Zam:
On the FFXIV Web site, the functionality of the system is illustrated through a fictional character named Leeroy. Here's what happens during a day in the life of Leeroy:
- Leeroy decides to try his hand in swordplay, choosing the discipline of the Gladiator. He goes outside to fight wildlife on the edge of his town's borders.
- Then some friends come and invite Leeroy to party with them in some nearby ruins. The party already has several accomplished warriors and mages, so Leeroy sheathes his sword and equips his staff, assuming the role of Thaumaturge, his most advanced class. He then joins his friend, fitting in with the party seamlessly.
- After the ruins party, Leeroy is following the river back to town when he suddenly feels the urge to fish. He puts away his staff and pulls out his fishing rod, taking on the skills and abilities of a Fisherman.
- When he finally gets back to town, Leeroy equips a hammer to take on the abilities of a blacksmith. Using his skills he goes to work sharpening his blade and pounding the dents out from his armor. All in a day's work!
From the first two points, we can see that players are allowed to change job in the field, but that doesn't necessarily mean one can change job "during a fight". I think there're certain restrictions may apply to changing your job, such as stats change and/or armor change.
We all know that FFXIV lets players to be able to redistribute their stats point freely, however, there maybe a cool down time in between. Since we have no infomation about that, I'm gonna take a wild guess. Let's say we can redistribute our stats points without any cool down when we're in our Mog house, so in that way, we can switch to any job anytime. When we're in the field, we can also redistribute our stats points (hopefully we can also set our stats point as macro, so even when we change job in the field, we don't have to spend too much time redistributing our points) with a cool down, so we can still change our job while we're in the field, but not as 'freely' as we're in town.
As for armor, I'm guessing we can change it anytime when we're not in active mode. So as long as we carry different sets of armors, then we can change it anytime.