Adjust min/max values when changing sequence type

Adjust min/max values when changing sequence type

When changing the type of a sequence, adjust the min/max values of the
sequence if it looks like the previous values were the default values.
Previously, it would leave the old values in place, requiring manual
adjustments even in the usual/default cases.

Reviewed-by: Michael Paquier <[email protected]>
Reviewed-by: Vitaly Burovoy <[email protected]>