The Archbishop of Canterbury and the Archbishop of York as well as the Bishops of London, Durham and Winchester are ex officio members of the House of Lords while the remaining 21 seats are filled (mostly) in order of seniority among the 35 eligible diocesan bishops (there are 37 but neither the Bishop of Sodor and Man nor the Bishop of Gibraltar and Europe may sit in the House of Lords as they do not belong to the UK).
Currently, the Archbishop of York’s seat is vacant following the previous holder’s retirement on 7th June 2020. A successor has been nominated but is not scheduled to be confirmed until 9th July. As this is an ex officio seat in the Lords, it is currently vacant.
The Archbishop Elect of York happens to be Stephen Cottrell, currently sitting in the Lords as Bishop of Chelmford. Thus, once he is confirmed as Archbishop, another vacancy will occur among the other diocesan bishops. This is to be filled by a writ of summons to be issued to the next eligible bishop which I understand to be the Bishop of Manchester. (Under the Lords Spiritual (Women) Act of 2015, any female eligible bishop would take precedence over the most senior male bishop until 2025; however, as far as I can tell there are currently no female eligible bishops. The act does not prevent male bishops from being summoned to the Lords as evident by three male bishops having become members since 2015.)
I suspect but I have not found evidence to prove or disprove, that filling the Bishop of Chelmford’s seat in the Lords will be effective immediately or next to immediately as it is well-known who is eligible and there is no female acting bishop whose appointment could complicate things. Thus, on 9th July I suspect we will see the membership list updated effectively to include David Walker, the Bishop of Manchester while Stephen Cottrell will remain a member albeit under a different title.