If I was attacking with a Transformers card that, on combat damage to a player transforms but said creature has double strike, will the creature deal first strike damage, convert and then deal regular damage with its converted side?
Yes, the creature would Convert (or Transform) before dealing the double-strike damage in the regular damage step.
From a ruling on Blitzwing, Cruel Tormentor:
After Blitzwing, Adaptive Assailant converts into Blitzwing, Cruel Tormentor, it's still an attacking creature in combat. It will still have any gained abilities, including the ability it gained at the beginning of combat. Notably, if it has somehow gained double strike, it will convert before dealing combat damage in the regular combat damage step.
This is because first strike damage and regular damage are separate combat damage steps, with triggered abilities being resolved between them.
The answer is "Probably, it depends" There are three possible cases here, in order of increasing likelihood:
- The creature blinks (exile and return transformed) when it transforms.
- The creature transformed in place but no longer has double strike.
- The creature transformed in place and what was giving it double strike is still there.
The first case here here isn't likely. While there are many cards that exile and return transformed, none of them trigger on combat damage, and only those that meld are creatures on both sides. Assuming you mean the Transformers card (BOT set) that convert, none of them including the 8 that can do so on combat damage exile to transform. If this did happen, the newly entered creature wouldn't even be attacking, and would not be part of combat.
The second is a bit of an edge case, but possible. If double strike was printed only on the front face (like Enduring Angel or the effect that gave double strike ended (say a soulbond with Silverblade Paladin who died when it was blocked), then the creature would no longer have double strike, losing double strike after the first combat damage step means the creature will not do damage in the second combat damage step, per the Comprehensive Rules (emphasis mine):
702.4b If at least one attacking or blocking creature has first strike (see rule 702.7) or double strike as the combat damage step begins, the only creatures that assign combat damage in that step are those with first strike or double strike. After that step, instead of proceeding to the end of combat step, the phase gets a second combat damage step. The only creatures that assign combat damage in that step are the remaining attackers and blockers that had neither first strike nor double strike as the first combat damage step began, as well as the remaining attackers and blockers that currently have double strike. After that step, the phase proceeds to the end of combat step.
The first case is by far the most likely, most transforming creatures don't blink when they transform, and effects granting abilities are tied to the object, not the specific form or face of that object, so if it had double strike from any ongoing effect like Duelist's Heritage, Fireshrieker or Boros Charm will still have double strike and still deal combat damage.