Did typical food for Roman soldiers while in the field change much during the period of the roman empire?

I've done a bit of research into this on account of historical recreation and it seems that, in all honesty, a soldier's ration really didn't change much from the Roman Era to the Civil War. A pound of meat (usually salted) and a pound of flour (in some form) was fairly standard.

Now, as to what this was, keep in mind that all depends. The Romans highly prized bacon and other fatty meats to avoid various nutritional diseases which could come from a lack of fat. Barley and wheat were commonly-provided grains in the Roman Era.

However, this really doesn't mean a whole lot. Your typical soldier is going to eat off the land as much as possible. Cheeses were very popular among the Romans (again, looking for fat) and the typical bacon was often supplemented with locally-sourced pork, beef, venison, etc. A salt ration was also provided and used to preserve meats for future consumption.

Posca is another thing worth mentioning. This mixture of vinegar, honey, water, and various herbs was, effectively, the Roman version of Gatorade. This mixture, albeit slightly changed up, is used all the way to the Revolutionary war (as switchel, haymaker punch, etc.). It provided electrolytes (they're what plants crave) and, depending on the age of the vinegar may have even contained a small portion of alcohol. A wine ration was also fairly standard in the evening.

Keep in mind, though, that everything is going to change based on where in the Empire a soldier is. If there's a lot of sheep, expect to see more mutton (I've never tried salt mutton) and less bacon. If they're somewhere in North Africa, I would expect more... Goat? Likewise if they are near a major port, fish (and garum) would be more plentiful.

Still, pound of meat + pound of grain, supplemented with salt and water and some form of alcohol (and whatever else can be found/scrounged/looted) is the basis of the ration for a very large portion of history.