Using values in raster to extend a raster/polygon to contour/LiDAR value

I'm using ArcMap.

I have a raster of flood levels (LiDAR + a water depth).I want to add 50mm onto the flood level raster which I can do with raster calculator.But then what I want to do is 'extend' the flood level raster out to the same level in the LiDAR

e.g. my flood level is 10m and the flood extent is 5m wide. I then add 50mm to the flood level and the LiDAR shows that a flood level of 10.05m would then be 5.5m wide.

How can I extend the raster flood levels to the corresponding LiDAR level?

I don't need the level values in the end output, just the flood extent so I can convert to polygon or whatever else.