I'm not sure this is the appropriate thread, but I just watched the third Fairy Tail OVA, Memory Days.
(Because I bought the special edition of vol. 31 that it came with).
It's called "Memory Days" and is a simple time-travel story where Natsu, Gray, Lucy, and Erza (and Happy) go back in time to about a year after Natsu joined Fairy Tail.
So, obviously it doesn't effect anything.
Spoiler:
BUT there are a couple of neat things that do matter. One, you see Macao's ex-wife (i.e. Romeo's mother). Her name is メロウ (Mellow?) and she seems to be a dancer at the guild.
Secondly, we see that Natsu got the scar on his neck (usually hidden by the scarf)... from himself.
Also, Lucy first learned about Fairy Tail because Lucy from the future rescued her from a runaway carriage
Other than that it was less fan-service filled than the other 2 OVAs, but still had a lot due to dressing up Lucy in a bunny girl outfit and Erza in a cat outfit for no real reason.