Short answer. The letters of administration are revoked once the will is proved and allowed. All the administrator's powers cease, they must surrender the letters and render an account, and the estate proceeds instead to letters testamentary or administration under the will.
What the law says
If after letters of administration have been granted on the estate of a decedent as if he had died intestate, his will is proved and allowed by the court, the letters of administration shall be revoked and all powers thereunder cease, and the administrator shall forthwith surrender the letters to the court, and render his account within such time as the court directs.
Rule 82, Section 1 — Administration revoked if will discovered; Proceedings thereupon. Read the full provision →
Proving and allowing the will is what triggers revocation
The consequence follows once the discovered will is proved and allowed by the court — mere discovery of a document purporting to be a will is not enough by itself. Only after the will has actually gone through the process of being proved and formally allowed does the earlier intestate administration get displaced by this particular rule of procedure.
A will that is contested and ultimately disallowed does not trigger this consequence at all.
Revocation is immediate and total, not gradual
Once the will is allowed, the earlier letters of administration shall be revoked and all powers thereunder cease. This is not a phased handover — the intestate administrator's authority to act on behalf of the estate ends as soon as the will is proved and allowed, not at some later, more convenient point in the ongoing proceeding itself.
Any acts the former administrator attempts after that point carry no legal authority behind them.
The former administrator has concrete duties to wind down
The administrator does not simply walk away. They shall forthwith surrender the letters to the court, and render his account within such time as the court directs. This accounting requirement ensures that everything the administrator did with estate property while acting under the now-revoked letters is fully documented and properly reviewed by the court.
The court sets its own deadline for this accounting, rather than leaving the timing to the outgoing administrator's discretion.
The estate then proceeds under the will instead
Once the intestate administration is revoked, proceedings for the issuance of letters testamentary or of administration under the will shall be as hereinbefore provided — the estate moves forward through the ordinary process for settling an estate that does have a valid will, rather than continuing under the earlier arrangement that had assumed there was none.
This effectively restarts the appointment process, now guided by whatever the will itself provides regarding an executor.