deprecation warning for bigtable gc policies creates impossible situation

[wyardley]

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Hello
We started getting deprecation warnings like this:

for bt gc policies, we got the following error:
Warning: Deprecated Attribute
  on bigtable.tf line 34, in resource "google_bigtable_gc_policy" "foo":
  34:     days = 3
Deprecated in favor of duration
(and 19 more similar warnings elsewhere)

That's fine, so Ok, we change 3d to, say, 72h. At that point, though, terraform apply returns the error
Error: doesn't support update if we try to a change like this

  ~ resource "google_bigtable_gc_policy" "foo" {
        id            = "(age() > 3d || versions() > 1)"
        # (5 unchanged attributes hidden)

      ~ max_age {
          - days     = 3 -> null
          + duration = "72h"
        }

        # (1 unchanged block hidden)
    }

We can remove the state and try to reimport it, however, then we find out that this is not an importable resource

Error: resource google_bigtable_gc_policy doesn't support import

It may be safe to simply remove the state, reapply everything, and not worry about dupe resources being left behind, but I don't know enough about bt garbage-collection policies to know if this is safe / sane. In either event, the current experience is frustrating.

Terraform Version

Terraform v0.14.7
+ provider registry.terraform.io/hashicorp/google v3.56.0
+ provider registry.terraform.io/hashicorp/google-beta v3.56.0

(tried with 3.57.0 as welll)

Affected Resource(s)

• google_bigtable_gc_policy

Terraform Configuration Files

resource "google_bigtable_gc_policy" "foo" {
  instance_name = module....
  table         = module....
  column_family = "data"

  max_age {
    duration = "72h"
  }
}

Debug Output

can provide if needed

Panic Output

Expected Behavior

When changing from, say, 30d to 720h for duration, we'd expect that the change will be a noop or state only change

Actual Behavior

Terraform gives the error

Error: doesn't support update

Steps to Reproduce

1. terraform apply

Important Factoids

References

[rileykarson]

Deduping to #8416

[ghost]

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