I've been using one varient or another of the same basic check register file for balancing my check book since filemaker 3. I don't know why if would fail to convert just from what you've described.
Things to try:
Run a recover on the file and see if it reports any problems in the file. Test the recovered file to see if it works. Some times one version of filemaker will tolerate file corruption that a different version does not.
Open up Manage | Database | Fields and look at the field definitions to see if anything looks out of wack. A basic running total summary field is set up just the same in FMP 11 as it was in FMP 3, you just have some new options added that weren't available back then.
Thank you, I'll try this and see if it works.