The word for untangling yarn or taking apart a yarn garment, 'unravel' (or 'ravel'), has an early modern origin. But people (especially, one imagines, the English) have been taking apart yarn garments to remake them since ancient times, and there must have been a much older terms for doing so.
For example what word would the historical (c. 1400) Richard II have used (and had he been drawing from concrete usage) in Shakespeare's (c. 1595) sentence:
Must I do so? and must I ravel out my weaved-up folly?
I would like to concentrate on wool and on knitted garments. As one commenter pointed out, an old woven garment would more likely have its seams picked apart, then been recut and resewn; the yarn would be less likely to be "unwoven".
Thus, what would term would a knitter (or a weaver of coarse, worsted fabric) in, say, Norman England, have used to describe (in the "vulgar tongue") this crucial step in reclaiming an old woolen garment for use in constructing a new one. What about even earlier (Old English)?
What are some Medieval words or Old English words for unravelling a knitted garment?
Support for this practice is given in Manufacturing Cloth from Wool: Medieval methods....:
While knitting wasn't wholly unknown in the Middle Ages, scant evidence of hand-knitted garments survives. The relative ease of the craft of knitting and the ready availability of materials and tools for making knitting needles makes it hard to believe that peasants didn't knit themselves warm clothing from wool they got from their own sheep.
The lack of surviving garments isn't at all surprising, considering the fragility of all cloth and the amount of time that has passed since the medieval era. Peasants could have worn their knitted garments to pieces, or they may have reclaimed the yarn for alternate uses when the garment grew too old or threadbare to wear any longer.