@DSmith95 HERE IS THE SOLUTION 
Try this using the v1 APIs. They are quite stable and still supported. The page itself quotes:
“Our APIs are in the midst of a transition. As part of our effort to improve API consistency and completeness, we launched the next generation of HubSpot’s API in early 2020. However, not every endpoint or API has been updated to the latest version, and our older endpoints are still stable and supported.”
Use this v1 batch contacts endpoint:
https://legacydocs.hubspot.com/docs/methods/contacts/batch_create_or_update
Use the following example request body
[
// assume existing email
{
"email": "test+old@hello.com",
"properties": [
{
"property": "email",
"value": "test+old@hello.com"
},
{
"property": "firstname",
"value": "User"
},
{
"property": "lastname",
"value": "Old Updated"
}
]
},
// new email
{
"email": "test+new@hello.com",
"properties": [
{
"property": "email",
"value": "test+new@hello.com"
},
{
"property": "firstname",
"value": "User"
},
{
"property": "lastname",
"value": "New"
}
]
}
]
CC: @Jaycee_Lewis - I see a lot of users in the community suffering from the same issue. Maybe you can suggest them with using this.