Preserve query when switching to Classic query

[Loori-R]

Description:

When switching to or from Classic query in a variable, the current query input gets cleared.
It would be helpful if Grafana preserved the previous expression - especially since Classic query supports equivalent formats used in other types.

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Example:

The following Label values config:

is equivalent to this Classic query expression:

label_values(flag{cluster="sandbox"}, job)

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Why it matters

• Prevents accidental loss of query data.
• Makes switching query types seamless.
• Saves users from retyping queries manually.