A gauge change alters how much each string stretches when you fret it, so intonation often shifts. Heavier strings usually need a little more compensation, which means the saddle moves slightly back.
Step by step:
- Put on the new strings, settle them and tune to pitch.
- Play the 12th-fret harmonic, then the fretted 12th-fret note. Use an accurate tuner.
- Sharp: lengthen the string by moving the saddle back.
- Flat: shorten the string by moving the saddle forward.
- Retune after every adjustment. Small turns are enough.
Use the guitar in the position you play it, with fresh strings. Old strings can intonate poorly no matter where the saddle sits.
Check action and neck relief before you set intonation, because changing either will move it again. Bigger gauge or tuning changes may also need truss rod or nut work. If a saddle runs out of travel, or you aren't comfortable adjusting the bridge, a technician can set it up.