Re-paint perhaps

Even indoor all plastic will eventually becomes yellowish. The common plastic PP, PE and PVC will turn to yellow faster than more expensive one like polyester, polycarbnate, etc. Most common cause is UV light more than heat inside house.

I can summarize from responses:

1) Change when no spare part to be found
2) Change to inverter for more power saving
3) Wear and tear will make the air-cond less efficient
4) Just change for something new.