This is a good question you ask and the answer is “yes”, but let me explain the 2 reasons why there might be an additional charge for full dates.
One reason is that the companies who manufacture the niche plaque (also known as a niche plate) can and do prefabricate the year, so for example, they have 1940 and 1980 pre-made before orders are ever placed and so those 4 digits for birth and death are usually included in the price of the plaque/plate when you purchase it.
To have the full dates, this is a custom order as there would not be any pre-made dates that include the month and day. This would be the case for a niche plaque that uses screws to attach the birth and death dates which are usually separate small rectangular plates.
The second reason for an additional charge is that if the niche plaque is all one piece, meaning no small plates are attached by screws for the dates, then additional charges (by the manufacturer, usually a bronze company) would apply for the full dates because there are more digits (letters, numbers) involved.
I hope this was helpful, but please reply if you have any further questions.