Editor Paul Kaminski says:
Sonic Universe celebrates 50 issues - and Metal Sonic is crashing the party! On the eve of the epic Sonic/Mega Man crossover, 'Forged in Fire' will star a newly-rebuilt Metal Sonic as we've never seen him before - consumed by vengeance. It's metal-brother versus metal-brother as the heroic Shard throws down with this new hatred-fueled Metal Sonic! Metal has been a cold, calculated killer since he debuted in comic book form back in Sonic the Hedgehog #25 - and that original appearance will also be re-presented as a special bonus feature in SU#50! We've also got a stunning new cover by Sonic-art legend Patrick "SPAZ" Spaziante and colorist Matt Herms which will sport comic-shop-exclusive metallic effects! The 'Sonic Underground' story originally slated for this issue in on a temporary hiatus, but fear not! There will still be music to celebrate this historic issue, only this time it's HEAVY METAL!
There wasn't a particular reason given for the change in plans, it's possible the new story could more directly connect to the upcoming Sonic/Mega Man crossover, but this is just speculation.
"Forged in Fire" will run 48 pages, which, for me, is pretty exciting since it's been years since Archie has done any double-sized issues for their Sonic series. Since it's more content than the average Sonic book, Sonic Universe #50 will sell for $3.99 (which is a standard price for most current comic book series) instead of the usual $2.99. It's worth noting that Kaminski mentions that Metal Sonic's original appearance will be "re-presented as a special bonus feature..." so we can probably take that to mean that the main story will run the standard length and the story for Sonic the Hedgehog #25, "Go Ahead... Mecha My Day!" will run as a back-up story.
Here's a look at the cover, illustrated by Patrick Spaziante and colored by Matt Herms!
The cover will feature a special foil-enhanced finish (think the current Sonic Super Special magazine covers), but it seems this foil finish will be a comic shop exclusive. No other variant covers have been announced at this time.
"Forged in Fire" drops March 2013, just before the Sonic/Mega Man crossover gets underway! Get hyped. You can read the full solicitation over at Comic Vine.