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You will have 30 days to transition to FileMaker Cloud and be off of FileMaker Server after going through the BYOL Process. If you are going to need longer, or run into problems with licensing, you may want to contact sales at 800-725-2747.
Make sure you work with sales to convert the VLA to an AVLA before you try to set up FileMaker Cloud. FileMaker Cloud BYOL only works with annual licenses (AVLA, AFLT, or ASLA). For more info, see Purchase options for FileMaker Cloud.