I am not aware of any tool that converts a lettuce project to a behave project.
Therefore, you probably need to do that by hand (at least initially).

• I assume that most of the work is in the conversion of the steps

DIFFERENCES:

• You should select an assert-matcher, like: assertpy, hamcrest or …
• Step parameters: behave supports a number of step-matchers. parse is the default one, cucumber-expressions is the newest. You normal make a decision which one is used per project or per step-library (as default step-matcher). All except the regex step-matcher have type-conversion support for the parameters.
• Hooks: Are slightly different compared to lettuce (if I remember it correctly)
• …

SEE ALSO:

• https://behave.readthedocs.io/en/latest/
• https://github.com/behave/behave.example repo (and its docs: https://behave.github.io/behave.example/ )
• Cucumber-expressions support (which covers some important aspects what you normally do in a new project): https://behave.readthedocs.io/en/latest/new_and_noteworthy_v1.2.7/#step-definitions-with-cucumber-expressions

NOTES:

• A comparison is pretty hard (by now) because the documentation website for lettuce is gone.
• The lettuce repo still exists: https://github.com/gabrielfalcao/lettuce