Update resource group with ARM template - cannot create Linux app service plan - azure

Question

I have a pipeline that is going to update my Azure resource group using an ARM template. In my resource group I have a Linux app service plan. When my pipeline runs I get the following error:
2020-02-24T09:13:47.8665770Z ##[error]The template deployment 'template-20200224-091344-94a5' is not valid according to the validation procedure. The tracking id is 'cba7977a-ed1a-4807-a8bb-d7387bb9eae1'. See inner errors for details.
2020-02-24T09:13:47.8673575Z ##[error]Details:
2020-02-24T09:13:47.8683161Z ##[error]ValidationForResourceFailed: Validation failed for a resource. Check 'Error.Details[0]' for more information. [{"code":"FreeLinuxSkuNotAllowedInResourceGroup","message":"Cannot create a Linux app service plan because there is a limit of 1 free tier linux app service plan per subscription."}]
2020-02-24T09:13:47.8695096Z ##[error]Task failed while creating or updating the template deployment.
I thought that the pipeline should only update my app service plan, but it seems that it is trying to create a new one. So I suppose what happens is that it creates a new one, and then deletes the old. However, it cannot do this because there's a limit of 1 Linux app service plan and one already exists.
So my question is, how can I make my pipeline only overwrite the old app service plan instead of creating a new one and deleting the old?
Here is the resource in the ARM template:
{
"type": "Microsoft.Web/serverfarms",
"apiVersion": "2018-02-01",
"name": "[parameters('serverfarms_project_api_name')]",
"location": "West Europe",
"tags": {
"Environment": "Dev"
},
"sku": {
"name": "F1",
"tier": "Free",
"size": "F1",
"family": "F",
"capacity": 1
},
"kind": "linux",
"properties": {
"perSiteScaling": false,
"maximumElasticWorkerCount": 1,
"isSpot": false,
"reserved": true,
"isXenon": false,
"hyperV": false,
"targetWorkerCount": 0,
"targetWorkerSizeId": 0
}
}

Answer 1

for the arm template to update existing resource - the resource name has to match exactly. that's the only trick. if the name matches - it will update the resource instead of creating a new one with the same name (not that it is possible).