My one big piece of advice as I mention above is take special care of the needs of the elderly. Literally dozens of times I have seen them just say I can watch Mass on TV now or pray at home.... My spiritual home where I grew up is gone. Go to be a great sorrow and broken heartedness in it that requires special attention.
I have also seen them brought back as their grandchildren receive the sacraments in the new combined Parish. It just takes a mourning time of sorts
Indeed, there were lots of parish closures in the UCC back in the day, and I saw precisely what you saw, albeit in a worse way, in that the low population density of some of the rural areas where the UCC operated as the main local church, combined with an aging membership and a relative lack of grandchildren or great grandchildren receiving the sacraments at the UCC (families tended to gravitate towards non-denominational megachurches, baptist churches, and evangelical churches). When the inevitable finally came, the trend in the UCC, owing to the historic nature of many of the churches, precluding their demolition and redevelopment, has been to sell surplus property to a mixed bag of denominations and independent churches, including community churches, “Bible Churches”, Baptist Churches (including Southern Baptist Churches, oddly enough), pentecostals, and small non-denominational churches, which sometimes brand themselves as “chapels,” for example, I know of a small non-denom chapel near LA called the “Sonrise Chapel” and another called the “Blessed Hope Chapel” - I believe this is to differentiate these smaller evangelical churches from evangelical megachurches, which share a common theology but differ substantially in terms of size and consequently, the number of programs on offer.
The problem faced by the UCC and the other shrinking mainline churches in the US again goes back to population density in rural areas. When a rural parish closes and gets taken over by a non-denominational type church, the result tends to be the alienation and de-churching of elderly people, because the distance to the next closest town with a traditional church in the rural parte of the US is very often too far to drive. I know of some Eastern Orthodox diehards who will commute two hours to go to an Orthodox church, but that is simply impossible for most older church attendees. So, when the parish changes hands, usually the worship also changes radically, with praise and worship music replacing the traditional hymns and organ music, if the church is so equipped, unless the parish is blessed with remarkably good luck and winds up being purchased by the SBC, LCMS, WELS, PCA, OPC, or a Continuing Anglican parish, because the first five churches often maintain traditional worship, and the continuing Anglicans always do.
Finally, while my post centered on the UCC, which by all accounts is the fastest-shrinking mainline denomination, they are all shrinking, even the massive United Methodist Church, albeit at a slow rate (however, when the impending schism occurs, I expect the portion comprised of African and traditional North American churches to have growing membership, and I expect the other portion, limited as it were to North America, to contract at a rate that is in the ballpark of the other receding mainline churches.
I need to stress that I am extremely unhappy about what is happening to our mainline Protestant churches, indeed, what has been happening arguably since the late 1950s, but which accelerated and intensified in the 1970s. Even when I grew up, the “reliable churches” were generically emumerated to me by my family as Methodist, Lutheran, Episcopalian, Baptist, Presbyterian, Congregationalist and Moravian (there was a great deal of love for the Moravians in my family, despite none of us being a member of that denomination, as far as I am aware; we were predominantly Lutheran and Methodist, with the Methodists being ethnically German and the Lutherans, ethnically Swedish (and thus, like many Swedish Lutherans at the time, members of the Augustana Synod, which sadly got swallowed up into the ELCA).