I agree with Yoshi about changing race "overnight" seems a bit odd. However, I think their should be a balance in being able to change races and being able to change only features/gender:

1. Changing Races: This option is only available if you want to change to one of the new races (Female Roe, Male Miqo'te). This should be a choice based on the following example. Imagine a player creating his new character, and wanted to play a Miqo'te. However, he/she did not want to play a female character for what ever reason, and decided to choose a race instead with a male option, e.g. Male Elezen. However, now that the game supports a Male Miqo'te, I believe it would be fair to give that player another chance.

2. Changing Gender/Features: This option should be available for everyone. Some people, including Yoshi, might think changing gender overnight may be extreme, though. However, if they are adding new facial, hair and other features to all the races, I know a lot of people might like the new options better. I believe this is a fair choice for everyone.