Untill Jebus adds support for custom fields like 'COMPOSER' and 'REPLAYGAIN_*', then i would suggest that you do the job with a batch file.
Save the following script to a file with a .bat extension and then drop your folder holding your FLAC collection on top of the batch file(it dosen't matter if the folder contains sub-folders also). Remember to first enter your specific path for your flac.exe executable located on your system(just after: set encoder=). This batch file will re-encode all your FLAC files to the newest version using -8 -V and then the -f switch forces the over-writing of the output-file so that you will keep the same file name(and the old file is deleted/over-written).
CODE
set encoder="C:\Program Files\FLAC\flac.exe"
for /R "%1" %%f in (*.flac) do %encoder% -8 -V -f "%%f"
CU, Martin.
