Simple answer yes you will have to upgrade the existing system to meet the visual notification requirements for accessibility and the fire code requirements for emergency voice alarm notification.
2018 IEBC The existing building requires you follow today's adopted code for an alteration 3 remodel which this sounds like it qualifies as.
904.2 Fire alarm and detection systems.
Fire alarm and detection shall be provided in accordance with Section 907 of the International Building Code as required for new construction.
904.2.1 Manual fire alarm systems.
Where required by the International Building Code, a manual fire alarm system shall be provided throughout the work area. Alarm notification appliances shall be provided on such floors and shall be automatically activated as required by the International Building Code.
Exceptions:
1. Alarm-initiating and notification appliances shall not be required to be installed in tenant spaces outside of the work area.
2. Visual alarm notification appliances are not required, except where an existing alarm system is upgraded or replaced or where a new fire alarm system is installed.
904.2.2 Automatic fire detection.
Where required by the International Building Code for new buildings, automatic fire detection systems shall be provided throughout the work area.
ICC/ANSI
701.1 Scope.
Communications elements and features required to be accessible by the scoping provisions adopted by the administrative authority shall comply with the applicable provisions of Chapter 7.
702 Alarms
702.1 General.
Accessible audible and visible alarms and notification appliances shall be installed in accordance with NFPA 72 listed in Section 105.2.2, be powered by a commercial light and power source, be permanently connected to the wiring of the premises electric system, and be permanently installed.
2018 IFC
1103.7.1 Group E.
A fire alarm system shall be installed in existing Group E occupancies in accordance with Section 907.2.3.
907.2.3 Group E.
A manual fire alarm system that initiates the occupant notification signal utilizing an emergency voice/alarm communication system meeting the requirements of Section 907.5.2.2 and installed in accordance with Section 907.6 shall be installed in Group E occupancies. Where automatic sprinkler systems or smoke detectors are installed, such systems or detectors shall be connected to the building fire alarm system.
Exceptions:
1. A manual fire alarm system is not required in Group E occupancies with an occupant load of 50 or less.
2. Emergency voice/alarm communication systems meeting the requirements of Section 907.5.2.2 and installed in accordance with Section 907.6 shall not be required in Group E occupancies with occupant loads of 100 or less, provided that activation of the manual fire alarm system initiates an approved occupant notification signal in accordance with Section 907.5.
3. Manual fire alarm boxes are not required in Group E occupancies where all of the following apply:
3.1. Interior corridors are protected by smoke detectors.
3.2. Auditoriums, cafeterias, gymnasiums and similar areas are protected by heat detectors or other approved detection devices.
3.3. Shops and laboratories involving dusts or vapors are protected by heat detectors or other approved detection devices.
4. Manual fire alarm boxes shall not be required in Group E occupancies where all of the following apply:
4.1. The building is equipped throughout with an approved automatic sprinkler system installed in accordance with Section 903.3.1.1.
4.2. The emergency voice/alarm communication system will activate on sprinkler water flow.
4.3. Manual activation is provided from a normally occupied location.