If you start with an 870 Express, can you "dress it up" at a later date and essentually have a Wingmaster?
Absolutely. I've polished steel, and with the right equipment it's not too hard.
What you'd need:
A new Light Contour barrel.
New bolt assembly.
Spring retainer.
Aluminum trigger assembly, complete.
New walnut stock and foreend.
(Total cost of parts, significantly more than a NIB Wingmaster, but you could sell of the old parts on eBay to recoup some of the cost)
Polisher, wheels, rouge, etc.
Sander, vibrating, with progressively smaller grit paper.
Dremel or flexible shaft tool.
Blueing kit.
Torch.
Requisite skills.
Time.
I have an Express. I think it's a great deal in a knockaround, very durable shotgun that will even work fine for trapshooting. But I sure like my hunting buddy's Wingmaster and my similar 1100, which is built and finished to the same standards as the Wingmaster. The Wingmaster is lighter (LC barrel matters), smoother and tighter. But I wouldn't throw the Wingmaster in a boat, even though it's durable. Too pretty.
There are a few different parts (barrel and mag spring are both retained differently, extractor is machined vs. MIM, stock is different wood and shape, trigger group is aluminum rather than plastic, etc.). But the other difference is internal and external finish.
If you want a good forever shotgun for $250 new (right now, a local retailer is selling them for $240 with rebate), the Express is a great choice. No matter what other expensive gun you buy in the future, it will always be a great "backup" gun to keep along on a hunting trip, loaner for a buddy or new shooter, HD gun, nasty-conditions hunting gun, etc. It's a GREAT shotgun, especially for the price. It's unbeatable for the price.
But it won't be a Wingmaster unless you're a crazed machinist with nothing to do during a long, cold Winter.
The Express is a deal for under $300. The Wingmaster LC is WELL WORTH its $600 price tag, as well.