Read the quality as well as the number
The symbols C7, Cmaj7, and Cm7 describe different harmonies. C7 contains C, E, G, and B flat; Cmaj7 uses B instead of B flat; Cm7 uses E flat and B flat. Some familiar ukulele shapes make these chords physically easy, but their musical functions are not interchangeable. Play a plain C chord followed by each version and listen to the changed color. Name the altered note so that the symbol connects to a sound rather than only to a convenient finger position.
Check the melody before substituting
Choose a short phrase and compare the written triad with a seventh version. A sustained melody note may blend with the added tone or produce a sharper tension. Neither outcome is automatically wrong, but it should be intentional. Keep the same strumming rhythm and volume during the comparison. If the recording clearly uses a seventh, try to preserve that color before simplifying for convenience. If it does not, adding one is an arranging choice rather than a correction to the original chord chart.
Connect the sound to its destination
Try G7 moving to C and listen to the arrival, then use the same idea in another comfortable key. Practice the transition slowly enough that the chord quality remains audible and the pulse stays steady. AudioChords can provide the detected symbols and ukulele diagrams, but unusual labels still deserve a listening check. Use seventh chords where their tension or softness supports the phrase. Keeping some plain triads can make the colored moments more distinctive. A good arrangement does not need the longest possible chord name on every measure; it needs the right notes for the melody and the musical direction.