If you have a Blistercoil Weird out and you Dragonshift it, won't it get the +1/+1 and untap so it would be a 5/5 flying dragon? My reasoning is that you cast Dragonshift, putting it on the stack, and trigger the weird's ability. The ability goes on the stack targeting the creature and since the creature itself is not killed it is simply changed the triggered ability will affect the 4/4 dragon
{Blistercoil Weird} , {Dragonshift}
You are right about the result, though not about the process. When the weird's trigger goes on stack, it resolves before the dragonshift. It doesn't target the weird, but untaps it and adds +1/+1 until end of turn. Then it is transformed in a 4/4 dragon, but the +1/+1 persists making it a 5/5
It would be a 5/5 dragon. Effects that set power/toughness to a specific number are applied before buffs. When the game checks the state of the creature, it would see its a 4/4, then see the +1/+1.
What if there were 2 inst/sorc played before {Dragonshift}? Would the +1s still be there? So a 7/7? Or a 5/5
If they were cast before dragonshift resolved, the weird would get the +1/+1 until end of turn, which means it keeps them when dragonshift resolves and turns him into a 4/4. He would get all the buffs he got before the dragonshift, like his own ability or {Overwhelming Stampede}'s buff. Beware that an instant or sorcery cast AFTER dragonshift resolves would not trigger his ability, since dragonshift takes all his abilities away. Also, in case you gave him any abilities through effects like the stampede's, he would lose them as well, so you probably should play it after dragonshift resolves if you want him trampling. Thirdly, cards that give abilities continuously, like {Akroma's Memorial}, would still make your creatures have the abilities after the dragonshift