BDE code needs changing, indeed, but is usually quite easy to change to SqlDb for SQL based BDE code.
For dBase based BDE you can use TDBF and even for Paradox based BDE code there is a solution.
What is your underlying database format? Then I can probably give you some advice on how to port the code.
Note that even for Delphi BDE is not only legacy but deprecated and not maintained.
Note that if you own a Delphi professional or higher, but pre-delphi 2010, you will have the BDE interface for WIN32 and that can be massaged to work with FPC in the usual way (add {$IFDEF FPC}{$MODE DELPHI}{$ENDIF} I have done that for D7 code on a system with BDE installed.
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I looked it up and what I actually did a long time ago was this at the program level and unit level, at the top:
{$IFDEF FPC}{$MODE DELPHI}{$DEFINE VER150}{$ENDIF}
This mimics Delphi 7 and also works for versioning by commonly used include file. Since compatibility with D7 is close to 100% this can save a lot of time. This define needs to precede the include file or added on top of the include file.
It is still not a guarantee, but works even better than just {$mode delphi} in a lot of cases and it works if you have the sourcecode of the BDE interface units. Note you also need to have a Delphi professional + license to legally use this trick profesionally.