Duty is not due if you claim end use exemption. That can be done on a one-off basis; your shipper should be able to supply a form. Make sure they know in advance because it is very difficult to get duty back if no claim is made at the time.
VAT will be due and must be paid before clearance, unless you have a deferment account. You probably don't or you would know this stuff already.
These are both paid on the value of the services/upgrades performed plus the shipping. Shippers generally charge a fee for this on top.
Make sure that the correct paperwork is done when sending the units out.....or you can't prove that they went out for service and will be charged as for new imports.
Again your shipper should be ale to help. You will need commercial invoices in the correct format.