The difference between 500L and 300L is not as much as you might think, especially when it's a very small percentage of your grain bill.
I think you could choose either option-- a full on replacement, or cutting it in half to compensate. If you wanted to be really precise w/ the color, what you could do is find a recipe calculator program, and enter in the recipe as it is written, and note the color of the beer that is generated. Then, get rid of the 300L stuff and add 500L stuff in the recipe generator and play with the quantity until the beer is the color you want.
In an irish red, the roasted barley is only supposed to provie a "hint" of roastiness, so I don't feel that you have to be extremely precise on how much of it is in the recipe to get a flavor you'll like. Cutting 1/2 lb into 1/4 lb isn't going to have a huge effect on the flavor. That said, I'd personally just use all the same quantities and sub out the 300 stuff with 500 and just know my beer might be a bit darker than expected. I guess what I'm trying to say is that you will be just fine with either option.
