I'm trying to keep my campaign as sandboxy and open ended as possible, letting the players drive the action. But, I also want to gently guide them into the modules wherever possible.

We've completed Eye for and Eye, and our newest party member is a troll slayer, and after his masterful use of Korden's Hammer to slay the Demon, they are all indebted to him, and want to help him track down a troll.

What I'd like to do is lead them into Stromdorf country, and to track a troll to the farm that the night goblins have taken over. From there, when/if they discover the fragments of the mapstone, that can lead them back into Stromdorf proper. What I am hoping is I can basically just move the third encounter to the beginning, and lead them into the Holtz farm, and conclude with the return of the necromancer. This seems pretty doable to me, and it allows them to walk away from Stromdorf and return later if they so desire.

Anyone have any experience changing the order, or there unforseen pitfalls to doing so? Nothing I can't hand-wave over, I presume, but I'd like to find those issues first.

Just wrapped up Gathering Storm. I did almost exactly the same thing. My group had fought some hard fights against single foes and to mix things up, I started them against the greenskin swarm on the farm. Thanks to the nature of the fragments granting visions, this smoothly led the party into town and began the process of unraveling the mysteries behind them and locating the other pieces. With a vision of each fragment, I let the players choose which to investigate next, saving the undead scene for when the players were stuck in town and not quite sure where to go next.

They ended up gathering the herdstone fragment last, so with the extra time and for a climactic final encounter the beastmen besieged the city to add an extra challenge smuggling the stone back into town... and then back out again.

Looking back on the experience, I just advise you to be careful. For our group it ended up being a looong module, lasting twice as many nights as expected and towards the end, the players were sorely tired of the adventure. Also worth noting, three of my five players (two not familiar with other editions of wfrp or the feel in general) were genuinely upset at the grand finale feeling they had put in so much time and effort just to lose and that all of the time was wasted. It's a response that depends on the player, but caught me off guard and is something to look out for.