Long time reader, first time commenting
I own a Lost World, Which I bought in 2005.
It has a Synchro-Form harness.
http://www.wildequipment.com.au/ex_harn ... ode=WELOST .I use it with the standard shoulder inserts and with Link Strap that connects the two Hip Wings.
I have never used the Uni-Fit harness. I also haven't seen the modern version of this. My sister does happen to have an older W.E. pack with the old Uni-fit hip belt on it.
Both harnesses are significantly different in how they would ride on your hips. My suggestion would be to go into a retailer that has W.E. packs and explain that you want a different hip belt. You have to buy it through a retailer regardless. Ask to try on the packs that they have on the shelf to feel what exactly what each hip belt feels like.
You're unlikely to find a Uni-fit hip belt in a shop though as they are mostly only used these days by school groups as they are stronger/less fiddly/less delicate. The Syncro-Form harness is a very interesting/unique harness system compared to others on the market. I would never advise anyone buy it with out trying it on first. I think more people are likely to hate it then love it. I feel it's also largely misunderstood and miss fitted by many retailers as it's such a complex harness. Which is unfortunate as it really is amazing at heavy loads (30+ kgs).
One small thing that I feel with the W.E. harness compared to softer harnesses such as One Planet, Mont, Macpac etc. is that the canvas face fabric takes quite a bit of getting used to. I sometimes had to tape my hips when it was new as it rubbed a bit on my skin. Then I again, I see this regularly happen to other people even in soft cushioned One Planets
I also agree with the suggestion that you should email Sea to Summit. It's surprising how many emails Ian Maley personally responds to. I once asked a question about how to fit my pack as none of the shops new much about it. The response I got from him was remarkably detailed. He genuinely cares very much for the quality and design of his gear. I got the vibe that is he was very anal about correct design and he is very keen for people to understand why a product is the way it is. The W.E. website reflect his passion and accuracy for detailed description of products.
My only wish is that they start to produce lighter weight gear. I know there are a few new light weight packs coming soon.
I hope that helps somehow.