An SLC-500 MSG that uses DF1, DH+, or DH485 protocol takes only 14 words of control file space, either with a portion of an Integer file or an MG file element.
An SLC-5/05 MSG that uses Ethernet requires 51 words of control file space.
If there are messages in the SLC-5/03 program that use Channel 1 (the DH485 network port), they'll be converted to use Channel 1 on the SLC-5/05 as well. RSLogix can't automatically convert the control data blocks, so it presents this error to force you to go re-configure the messages to use properly sized data blocks.
One workaround would be to change the SLC-5/03 MSG instructions to use Channel 0, then convert the program.
Another workaround would be to delete and re-enter all the MSG instructions.