Realistically They Do What The Army Does
Look at George Washington and the Continental Army in the Revolutionary War/American War of Independence. Washington, along with a handful of other "Trained military men" from the British and other armies did not, by and large, try to adapt their tactics to their essentially-untrained troops. ESPECIALLY not in day-to-day military life. They drilled, they marched, they fortified their camps and posted guards in the approved manner of the British army at the time. At least, as close as they could get with untrained men. There were some exceptions to this (Nathaniel Greene comes to mind) But Greene was NOT someone with prior military experience.
Your troops would likely live, train, and march in the same style as the senior officer/drill officer's old army. Whatever level of discipline the former military men's armies had would likely be a little more lax in the rebel force, but otherwise "camp life" would be largely similar. The reason is that the officers Believe in the methods of their old armies. They've won battles and wars, maybe had centuries of success, based on the training and tactics those armies taught your officers. Your general comes from an army that builds a fortified camp every night a-la the Roman Legions? He is going to want to do that even IF it isn't necessary for the situation. Because that's what winners do. Yeah maybe they don't fear attack. But by god spending hours a day making a ditch&wall encampent builds character! Makes a soldier out of you! It's what we did in the Vandalian army and they haven't lost a war in two centuries so it obviously works! etc and soforth.
What's more, this band of peasants, unless it's totally removed from all armies ever, is going to have some sort of expectation of soldiery. If they know General Zhau's old Vandalian army marches in lockstep and keeps perfect alignment when delivering a volley, they're going to WANT to do that. Because, again, that's what soldiers do and it works for THAT army and doing something "not that" is obviously inferior until proven otherwise. And in war people are VERY reluctant to stray from tried-and-true methods, because that's how folk get killed. Granted sticking to the old ways when new tech/tactics come into play can also get you killed, but militarily speaking there's a lot more forgiveness for trying the Old Ways and failing than trying some Newfangled Idea and failing.
The last thing to consider is the officers themselves. They know (via having done it or via witnessing it) how new troops were trained in the Old Army. It's almost certainly the ONLY way they know how to train troops. So by definition if you train troops exactly the same way as their previous army you're going to get troops (and army life) that mimics that previous army closely.
Now over time you're going to see one of two things (provided the rebels don't lose). Either this professionalism wears off on the troops to the point that they're largely trained and fight to an equivalent level of their foes (a-la the Continental Regulars by Yorktown) or the difference in battlefield terrain/strategic position is so different between the Officer's old army's way of doing things and reality of the rebel army's needs that the old officers are shunted out of command to make way for soldiers who produce results.
Zoboso made a good point I want to expand on. There is some chance that your Medieval officer class only knows about a certain aspect of Medieval warfare. In Europe a lord might have general knowledge of everything, because he's expected to bring a mixed force of cavalry/infantry/archers to battle with him. but in China and elsewhere at the same time an officer might only lead infantry, or horsemen, or siege equipment. If your army lacks, say, Cavalry officers, the peasants may end up with wildly different cavalry tactics as the "real" officers gravitate towards what they know and leave room for leaders without military experience to command other forces. So, say, the Cavalry might end up WILDLY different than the "normal army" because all your regular officers were in the infantry.