Im not sure if we support this kind of upgrade, but here are the steps I used for reproduction:
1) Open HDRP template on 2019.4.18f1, it has HDRP 7.5.2
2) Open the project with Unity 2021.2.0a19 HDRP master branch

On open I found some errors but didnt see anything actually not working. Here is the stack trace:

NullReferenceException: Object reference not set to an instance of an object
UnityEngine.Rendering.HighDefinition.HDProjectSettingsProxy.get_projectSettingsFolderPath () (at /Users/motiejus.viltrakis/Documents/Git/Graphics/com.unity.render-pipelines.high-definition/Runtime/RenderPipelineResources/HDProjectSettingsProxy.cs:13)
UnityEngine.Rendering.HighDefinition.HDRenderPipelineGlobalSettings.MigrateFromHDRPAsset (UnityEngine.Rendering.HighDefinition.HDRenderPipelineAsset oldAsset) (at /Users/motiejus.viltrakis/Documents/Git/Graphics/com.unity.render-pipelines.high-definition/Runtime/RenderPipeline/HDRenderPipelineGlobalSettings.Migration.cs:61)
UnityEngine.Rendering.HighDefinition.HDRenderPipelineAsset+<>c.<.cctor>b__97_15 (UnityEngine.Rendering.HighDefinition.HDRenderPipelineAsset data) (at /Users/motiejus.viltrakis/Documents/Git/Graphics/com.unity.render-pipelines.high-definition/Runtime/RenderPipeline/HDRenderPipelineAsset.Migration.cs:174)
UnityEngine.Rendering.HighDefinition.MigrationStep`2[TVersion,TTarget].Migrate (TTarget target) (at /Users/motiejus.viltrakis/Documents/Git/Graphics/com.unity.render-pipelines.high-definition/Runtime/Core/Migration/MigrationStep.cs:52)
UnityEngine.Rendering.HighDefinition.MigrationDescription`2[TVersion,TTarget].Migrate (TTarget target) (at /Users/motiejus.viltrakis/Documents/Git/Graphics/com.unity.render-pipelines.high-definition/Runtime/Core/Migration/MigrationDescription.cs:111)
UnityEngine.Rendering.HighDefinition.HDRenderPipelineAsset.Migrate () (at /Users/motiejus.viltrakis/Documents/Git/Graphics/com.unity.render-pipelines.high-definition/Runtime/RenderPipeline/HDRenderPipelineAsset.Migration.cs:334)
UnityEngine.Rendering.HighDefinition.HDRenderPipelineAsset.OnEnable () (at /Users/motiejus.viltrakis/Documents/Git/Graphics/com.unity.render-pipelines.high-definition/Runtime/RenderPipeline/HDRenderPipelineAsset.Migration.cs:340)
UnityEditor.EditorAssemblies:ProcessInitializeOnLoadMethodAttributes() (at /Users/bokken/buildslave/unity/build/Editor/Mono/EditorAssemblies.cs:147)