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You can open FMP11 with FMP9 with no issues.
The file format of Filemaker databases changes occasionally. Previous versions can not open newer File Formats.
FMP V3 and V4 could open FP3
FMP V5 and V6 could open FP5 and convert FP3
FMP V7, V8, V9, V10, and V11 can open FP7 and convert FP5
FMP V12 uses FMP12 and can convert FP7
- .fp3 since FileMaker Pro 3.0
- .fp5 since FileMaker Pro 5.0
- .fp7 since FileMaker Pro 7.0, (including FileMaker Go 1.0)
- .fmp12 since FileMaker Pro 12