5e allows you to break up your movement and attacks.
BREAKING UP YOUR MOVE
You can break up your movement on your turn, using some of your speed before and after your action. For example, if you have a speed of 30 feet, you can move 10 feet, take your action, and then move 20 feet.
MOVING BETWEEN ATTACKS
If you take an action that includes more than one weapon attack, you can break up your movement even further by moving between those attacks. For example, a fighter who can make two attacks with the Extra Attack feature and who has a speed of 25 feet could move 10 feet, make an attack, move 15 feet, and then attack again.
I take this as implying that you can change as things develop. You could move and make one attack then make decisions based on the effects of the first attack with the rest of your move and attacks on your turn. I extend this to other things like "Surprise, 15 feet in you hit a pressure plate, you have 15 feet of move left, do you continue and take your foot off the pressure plate or stop?"
The way I would see it is a fighter declares he moves up and attacks an orc, this gets resolved, then depending on the resolution, he attacks the orc again with his second attack, or moves further to engage a second orc with his second attack.
This fits with my style of DMing, I prefer to resolve things sequentially as they happen rather than keep an intended sequence in mind and resolve discrepancies and make judgment calls of how to apply the character's declared actions to changed circumstances.
There is no explicit statement on whether a full round's actions for a character's turn must be announced at the beginning of the turn and executed regardless of changed circumstances. So a DM could rule that things cannot change after declaring your actions. That would be a more older edition style of handling round by round action, when everybody declared actions in the fog of war before rolling initiative for the round and planned actions could be wasted by developments in the round. All around as DM and player I prefer the more modern immediate control of your actions when your turn comes around and you are making decisions about the situation at hand as opposed to making decisions on how you expect things to be when you get to them. It makes sense to me to handle the character's actions in the round as they proceed, so they move their miniature, and the DM interrupts and stops them if they hit a hidden bear trap or whatever during their move.
Things get a little dicier in this opportunity attack situation because opportunity attacks once triggered, are resolved before they are triggered. As described this is a literal time travel and can create a paradox of preventing the trigger. I just take it as a mechanical resolution of the abstraction of time (everything is happening simultaneously but we break it down into distinct sequential turns) and action in combats. Narratively I treat it as being triggered at the resolution point, right before they leave the square when they are still in reach for the attack. Mechanically it is triggered when they leave reach but then time travel rolls back time for the opportunity attack resolution when they are still within reach. In either case in my view after the attack they are then at the point after having been hit by the opportunity attack and can proceed with their decisions from there with what move and actions they have left in the round and from that position (still within reach).
I could see a DM reasonably ruling they cannot voluntarily abort their movement to the square outside of reach. I could also see a DM saying the cantrip effect comes into effect in the new square outside of reach so they can react to it with a decision as if they were hit on an opponent's turn. I could also see a DM doing the fog of war thing and going with the full movement declaration and so no chance to avoid the booming blade movement damage.
My preference, and what I think is the more natural implications of the movement stuff the PH presents is that combatants can adapt to different situations mid round and redeclare how they use any as of yet unused movement and action in the round.
There is plenty of room for different DM rulings here as the specifics do not seem to be covered by the rules directly.
