Okay, I hope this isn't a repost, tried searching, but didn't find anything.
If you have a pet summoned which requires Soul Shards to summon, you can simply press "Stay" on your pet hotbar, and walk around 50 feet away, and you will have lost your pet, but regained your Soul Shard! This way, you can basically summon whichever pet you please if you have one out already.
Hope this was useful