It will depends on the Terms and Conditions preset in your current Tenancy Agreement. Most agreement will provide a clause stating that the current deposit is NOT to be used to offset any rental. You might be at a risk should your current tenant decide not to extend their lease and leave u with an office with defects. The deposit is meant to provide a security blanket should there be any damages to your unit.
You can still keep the deposit and get the tenant to top up the deposit based on the new rental agreed by both parties upon signing of the new Tenancy Agreement.
The tenant should inform u should he have an interest to extend based on the terms and conditions preset in your current agreement.
Do contact me at email@example.com or 8298 3379 if u have more queries.
ERA Realty Network Pte Ltd