It might work out nicely if you go to the Table, delete the original field and then immediately build the new external lookup and ensure that the end result generates a field with exactly the same name as that which you'd just deleted.
Don't revisit the summaries between deleting and rebuilding.
I'm not sure why you need to make an external lookup in order to simplify the "updating of the model". Since Monarch v9, you can import calculcated fields and other objects from one model to another. You could have one model file - "Master.xmod" - and import (with strength "none") the lookup field to that, then delete it from the original model, and create a "strong" import to fetch the lookup field back in whenever the model is run.
I think that you may have misunderstood the intent here Olly.
I don't think that the purpose here is "simplify the model", but instead is to simplify the process of updating the lookup data.
Moving the lookup process from a lookup calculated field that is maintained within a Monarch session to employing an external lookup that can be maintained by peers without Monarch makes it easier for the lookup data to be maintained.