I would advise calling and asking them in advance. I changed my lights when I had a mkIV, and they charged no extra; but they did note it down on the wording in the policy details. They may take a view that it could make the car more attractive to theives?
In any case it's important (and in your own interests) for your company to be aware of any changes from original spec; whether that's OEM or otherwise. That way they can't try and weedle their way out of paying up in event of an accident / claim etc.
A friend of mine used to work in insurance, and there are some companies out there who will look for any excuse to get out of paying up; and as he said they have inspectors / loss assessors who know exactly what to look for regarding changes to cars; mapping included! If they note unnotified external changes; that's likely to increase their interest in looking deeper should they choose too.
Hopefully for tail lights you'll not get charged anything though.
