I guess to start with High Elves, there really isnt much more to know about them other than they are culturally, more or less, the same as Blood Elves but simply with a different political alliance. Blood Elves are just renamed High Elves from when Prince Keal’thas returned home after the destruction of Quel’thalas and followed him from Warcraft 3 through the start of Burning Crusade, joining the Horde. Those that retain the name High Elves are those who, for whatever reason, retained their Alliance ties and didnt take part in Prince Keal’thas’ attempts at saving his people or didn’t wish to join their cousins in the Horde. Aside from the green, and now potentially gold, eyes thing the difference between a high and blood elf, imo, is merely geo-political. It is a self imposed identity. These are the easiest non-Night Elves to play within the Alliance because the history is already there.
The best opportunity I can think of for a Blood Elf defector really is the BC-Cata era, BC being the easiest or the next best would be Legion when the Void Elves show up. As Im sure you’re aware Void Elves are all just voided Blood Elves, none to me knowledge had any High Elf associations.
If you want to go a BC-Cata route you can largely attribute it to being a Blood Elf that came to Outland either as apart of Keal’thas’ forces or the Hordes and eventually joined the Aldor, Scryers, Sha’tar, or Shattered Sun Offensive, while being uneasy about the Horde given these same orcs allied with the Amani to destroy your home just 20 years earlier. Recalling your, albeit loose, Alliance ties cause you to think about rejoining your High Elf cousins and what that would look like. Major plot points could Velen and the Dreanei, the Alliance, saving the Sunwell, it could be hearing about the Wrathgate and the Forsaken, a final straw could be Garrosh and his wars, or Theramore. How you tie into any of those can be up to you and what your character was doing all that time.
With the Void Elves joining the Alliance, in my mind you’d need to have some connection to them. The only reason the Void Elves are able to join the Alliance is because Alleria. Voluntarily joining the Alliance, becoming a Hero of the Alliance, having a personal relationship with Turalyon, and being gone for 10 years, and learning to control the void, all contribute to the ability for her to say these new purple elves are safe. The Void Elves that have joined the Alliance would need to vouch for your character in some time, that they can be trusted or are safe to join. Perhaps your character is someone that never truly trusted the Horde and had old acquaintances who were apart of the Voided individuals to take them in, or a would be Void Elf but just happened to miss school that day. Those are the first hooks that come to mind.
I think a fun fight you can have is what does being a Blood Elf mean to you/the character? The name Blood Elf was made to reflect the pain and suffering the High Elves experienced. It is a very purposeful name that can mean a lot but does not have a long history. One would only really have adopted that name for 20 some years after being known as a High Elf or potentially 100s. What do these identities mean to you? Do you even keep calling yourself a Blood Elf? Do you have any dead give aways like still green eyes? How green is green? Just things to consider.