I have a dilemma for my user created (original) High Fantasy setting. I have every zone, NPC, region and it's potential creatures all designed and solid. It's been a long running RP over Forums for a long time. I'm not lacking in story or background or how to tell said story. I am telling said story over a 'Virtual Tabletop' which I wont advertise on here. Suffice to say it allows me to physically build my maps and encounters with tokens and tiles and such. To pre-build the next leg or 5 of their journey. It's immersive...and they love it so far.
My problem comes from being a semi fledgling Dungeon Master and having no idea how to go about allowing them freedom of going where they wish and keeping the creatures they may encounter in any given area static with their level. Even with the full DMM and MM at my fingertips, I can't find a way (that I understand) to take a set of encounters I pre-built for them to go through at level 4, and turn them into level 6 encounters because they went a different path and will later arrive at said encounters.
I'll try and elaborate without rambling.
I originally thought they would go the following route:
Greater Forest Starter - Lesser Forest - Plains - Mountains
I built encounters to their EXP Threshold to make sure they had smooth leveling experiences as they went through. Well the above plan didn't go as I thought and they'll be going north straight to the mountains first as they seemingly pose the most intrigue to them. I have NO problem with this....but now I have an entire map of encounters built for level 6 that need to be level 4. No where can I find an equation or chart on how to alter creatures from the Monster Manual (MM) and make them higher or lower to fit my exp budget needs for them. 4E had an online monster creator and easy methods of making my own. 5E? Not so much.
I don't want to railroad my players into a linear adventuring experience if I don't have to. What are some suggestions to encounter building encounters that are easily changeable either up or down pending player character choice?