NAV SQL Transaction Log Backup

Have a NAV 4.0 SQL customer who did not back up their SQL Transaction Log for a long time. Now it is very big and when they try to back it up so all completed transactions are removed, there is not enough disk space to do so. Any ideas how to recolve that without taking any risks? Seems like it got very big very fast because they by mistake activated NAV Change Log for all fields.

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