Can confirm it's possible for avios upgrades to J to clear even if there is no U availability, but as others have already advised it's by no means a guarantee and be prepared to fly Y. Presumably, the emptier the business cabin - the higher chance revenue management could consider reallocating a cash fare to U, but the reality is many more factors come into play. Would recommend you also consult the 'Avios Upgrade Eligibility' thread on the first page, where I've gone into detail around my recent experience with avios upgrades.
Just as a note, if your avios upgrade does clear, be prepared to:
- no longer be able to access/manage your booking online (best ring them to ask for a particular seat, e.g. throne, asap)
- avios upgrades are non-transferable and non-refundable, regardless of the original fare conditions of your Y ticket. If your plans change, you lose the points and the upgrade does not carry across.
- TCs are earned according to your original Y ticket cash fare class.
- award or upgraded J tickets (be it avios or voucher) are not eligible for a companion ticket in J for example.