Can a serving IAS/IPS/IFS officer re-attempt UPSC to improve their service?

From CSE 2026 onwards, NO — serving IAS and IFS officers cannot re-appear at all unless they resign first. IPS officers can re-attempt but lose their IPS permanently. A one-time grace exists for those allotted via CSE 2025 or earlier — they may try in CSE 2026 or 2027 without resigning. From CSE 2028, resignation is mandatory.