There's potentially three spots up for grabs based on tonight's line up (if Brouwer's exposed it's because they found a better right winger to protect, which would still leave three spots)
Tkachuk-Backlund-Frolik
Gaudreau-Monahan-XXX (formerly Versteeg)
Ferland-Bennett-Brouwer
XXX-Stajan-XXX (formerly Bouma and Hamilton respectively)
Trading Bouma for Colborne takes up one of those bottom two spots which I'd much rather see available for a prospect to win if they're good enough. The other fourth line spot should be a rotating competition between Hathaway (who'll need to go through waivers next year), Bouma (if he's still around) and Hamilton (ditto). Acquiring Colborne essentially guarantees him one of those spots because the threshold for a benching or waiving is so much higher than it is with Bouma.
Versteeg's spot is where they should be looking for a permanent upgrade, e.g. Oshie, Bjorkstrad, Radulov, Ritchie, Williams, McCarron, Silfverberg, Baptiste, Pastrnak, McCarron, Scherbak, etc. If they find that better right winger before the expansion draft, then they'll have to expose Brouwer, who's hopefully taken freeing up a roster spot. I'd much rather see that available for a Pribyl, Mangiepane, or Jankowski (move Stajan to the wing) than essentially waste it on Colborne. And with the Flames needing to sign two goalies, four defencemen along with Bennett, Ferland and perhaps an expensive right wing free agent, they'll need to conserve all the cap space they can, which may necessitate moving Bouma's salary just to make things work. So the idea of adding salary with Colborne is even more impractical. And if none of the prospects are ready, the Flames can always find cheap stop gaps like Versteeg, Hemsky, Parenteau, Eaves, Wingels, Iginla, etc., at the last minute.
Bottom line is that there's no need to revisit the Colborne show. It was done, it was wholly forgettable, it's time to move one.
What this organization has kind of failed to grasp over the years is that competition works both ways. Veterans ensure that prospects need to be better to win a spot but prospects force veterans to be on their toes if they want to keep theirs. That dual competition is necessary to prevent complacency from the replaceable periphery guys (Brouwer, Stajan, Bouma, Versteeg, Wideman, Engelland, Jokipakka, etc.). That's why there needs to be spots available on a regular basis to foster that competition rather than filling them with mediocre retreads and placing the burden entirely on non-phenom prospects to displace millions in salary from a penny pinching ownership.