What is the difference between 呀 and 啊?
ABC's entry for 呀 says:

呀
ya5
(replacing a (啊) when preceding word ends in a, e, i, o, or (y)u)
快来呀!
Kuài lái ya!
Come quickly!

呀 is a replacement for 啊 when it comes after a vowel.
This is also corroborated by《规范》's entry:

助 “啊(a)”受前一个音节末尾音素ɑ､e､o､i､ü的影响产生音变而采用的不同写法
说话(huà)呀 | 车(chē)呀 | 你说(shuō)呀 | 起来(lái)呀 | 快去(qù)呀。