A redeco, or recolor, is a toy which uses the same molds as a previously-released toy, but has been reproduced with different plastic colors and/or different paint applications.
A redeco uses exactly the same tools or molds as the original toy. If any changes have been made, even something as simple as giving the toy a different head or additional pegs for locking body parts into place, it is more properly categorized as a retool.
The term "repaint" is also in wide use among fans as a synonym for "redeco". "Redeco", however, is the term used by the people on Hasbro's Transformers team. Additionally, the term "repaint" is technically wrong in almost all instances of a same-mold-new-colors toy, as typically more than the paint applications are changed when a toy is redecoed.
The single most expensive step in making a Transformers toy is making the steel-cut molds required to hold then cool molten plastic. Add to this the salaries of the designers and engineers, and there is a very considerable expense involved in making a single new-mold toy. Many of these development expenses are not involved in making a redeco, giving the company a chance to make a larger profit on a single mold and put more product on the shelves with a smaller investment.
Another reason is that the toy market moves far, far faster today than it did when the line first started. Retailers do not like product to linger, and shipping older toys in new assortments can give the impression that some items simply do not sell. (Of course, some don't anyway, but that's beside the point.) With this much shorter sales window in mind, companies use redecos to keep molds in use longer (to help make back the money spent in development plus a worthwhile profit), and oftentimes keep popular characters still in circulation while maintaining a "fresh" look on the shelves, making the retailers happy.
Redecoes as exclusives
Most exclusive toys, be they for conventions or stores, are redecos and retools of previously-released toys. (With just one exception, all exclusives that used previously-unreleased molds are toys that had been developed for normal retail but were temporarily in "limbo" when their intended toylines were ended. The one exception, Aveo Swerve, was a new mold produced for a gigantic multinational corporation.)
The simple reason for this is, again, cost; making the steel molds is prohibitively expensive, as well as the engineering and so forth. Because most exclusives are produced in numbers far less than a normal retail toy, there are less units to amortize what production costs do remain, which means the toys must either sell for more to cover costs and make a worthwhile profit, or the profit margin per item must be reduced, or even both at once.
While it is certainly not outside the realm of possibility that a major retail chain might get a new-mold toy as an exclusive (as they can move thousands and thousands of units nationwide over several months), the odds of a convention getting one are almost nonexistent due to these cost considerations, as the production runs of these toys rarely reach even 1000. At most, therefore, one can expect a few retools of select parts from a toy, often a new-mold robot mode head.