We recommend this the sequence of steps:
Create a new custom field Company new with CRM Company type.
(!) While creating you will need to add any one option to Options field. It doesn’t matter what option will you add, it will not be shown anywhere. It’s just we not invented yet how to skip this step.
Place this new field to all screens where there is the old Company field.
Go to the new field Settings.
Press «Import values…»
Then press «Migrate values…".
Check one-two issues that the new Company field value is the same as the old Company field value.
In case you have autocomplete set for field Company, set it also for the new field.
Rename the old field to Company (Old).
Rename the new field to the name that previously had the old field.
Now you may delete the old field from screens. We recommend to not delete it from custom fields or from field configuration to prevent possible errors.
As the new field has the old name it’s not necessary to change any saved filters, but still you may need to reset some gadgets and reports and agile boards.