Geraniums

My geraniums have been flowering up to early December in an unheated garage. For some reason they did not want to go into rest. They have not been watered, the flowers are now faded and some stems are starting to dry up. They are still in pots with (dry) soil. How soon before the average last frost date should they be repotted with fresh pottting mix? If not repotted now, would watering make them rot? I'm in east TN, zone 7, last average frost date April 9.
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