So... I've completely changed all of Dr. Mario except for the small area of the title screen that has the player select and copyright info, but I seem to have hit a brick wall.  Changing ANY tiles within that space seems to look fine, but then once I start the game it freezes after the first pill drops.   Just to give an example of what I am talking about, here are a few screenshots of changing the copyright date to an arbitrary number.

Original:


Copyright year changed, game freezes:


Aesthetically, it looks like everything is fine, but like I said before, as soon as the game starts and the first pill drops, the game freezes.  I have completely overhauled every other screen in this game with no issues whatsoever, so I am at a complete loss as to how/why this is happening.  I would SINCERELY appreciate any help that any of you more experienced guys could offer.  The addresses that I changed in that example are 0x003ca6-0x003ca9, although it does the same thing when I change any of the player text on the lines above it.  Again, any help at all would be greatly appreciated.

Might be a checksum issue. Try rearranging bytes rather than replacing them (i.e. try "9901" on that date) and see what happens.

Yep. Try 9109, and it stills run. Probably a checksum (likely for copy-protection).

the way around it is to find the checksum routine and disable it, or find the checksum value and use a new correct value.. but to do that you will have to know how the checksum works and if you know that you might as well just disable it

Wow!  You guys are great... I asked a question less than 5 hours ago and I've already gotten a bunch of helpful replies.  Thanks a ton for all of the info... I never would have figured out what the problem was on my own.

Anyway, I changed the value that rveach pointed me to, and so far all of the changes I've done to the title screen don't make the game freeze.  What worries me is that I simply changed the number randomly from c9 c1 to b9 b1.  Oh well, I guess I shouldn't look a gift horse in the mouth.. haha.

Seriously, you guys are awesome.  Thanks again.

I wouldn't go around changing opcodes randomly. You just got lucky with the opcode you changed it to isn't anything serious. Otherwise you could crash or cause the game to really mess up.

If you know how to set breakpoints, set one at 91EF and see what the new checksum value is that is stored in $49 and change the C1 in "C9 C1" to that new value.
Otherwise doing what Ryusui said is fine.