Q: If you unlock the Sensor Module in the Pilot Tree (500GXP) and then buy the Sensor Module in the Mechbay (2,000,000 C-bill), what happens when you later unlock the Advanced Sensor Module (7500 GXP)?
A: As of Patch 1.2.182, the Sensor Module loaded on your mech automatically upgrades to the Advanced Sensor Module. No need to spend another 2,000,000 C-bills.
I suspect this is true of the other multi-stage modules (e.g. Target Decay), but cannot confirm that personally (yet).
Hopefully this helps somebody else who's trying to weigh this investment.
Edited by Lentil, 16 February 2013 - 08:20 PM.