If you have added something original, non-obvious, and useful that improves the original invention, you may be able to obtain an improvement patent. But, beware: an improvement patent will not give you the right to manufacture and sell your improvement. To do that, you will need to obtain a license from the original patent holder (unless that patent has expired). However, it will prevent the original patent holder from manufacturing or selling your improvement unless it obtains a license from you. Often, the holders of the original and improvement patents will cross-license so each may manufacture and sell both the original and the improved invention. Patents are a very complex subject and you should not attempt to operate in this area without the advice of a licensed patent attorney.
Creating a MAP will take no more than an hour of your time every month and will keep the lines of communication open, ensuring relationships with investors remain strong, and ultimately helping early-stage startups succeed.