I wouldn't be surprised if UA already swapped part of their -10 order to -8's. The MAX-8's for UA now rolling out at Renton were already in production before UA firmed their MAX-8 order this year...
Boeing probably won't say anything about it. UA swapped a substantial part of their launch order for 787-10 to -9's a few years ago, this was also done quietly. As Boeing doesn't disclose which version of the MAX is ordered anyway, we'll probably only know if we ever see a 737-10 in UA colors...
United's MAX 8 order has been on the books for a few years now. People started talking about the MAX 8 order in 2019 (full disclosure it did take me a while to find the order and I had to eat a slice of humble pie on the United Fleet/Network thread in 2020 when I finally did locate some information internally that confirmed UA's order).
Why would UA get rid of their MAX 10 order?
The 50 A321XLRs will most likely be used out of EWR, IAD and maybe ORD to secondary European destinations and perhaps IAH-to select destinations in South America. Also United has already stated the A321XLR will have some type of premium cabin with so I'm estimating UA's seating capacity on the A321XLRs to be between 150 and 160 total seats.
Right now United has a major shortage of narrowbody aircraft in the 185-200 seat range. Our 739ER/MAX 9 seat 179 total passengers, after the 739ER/MAX 9 we have nothing except 753s at 234 total seats and we only have 21 753s in our fleet. Now compare United's narrowbody fleet on the high end to that of AA and DL and what you will notice is both these airlines have a whole slew of A321s that can accommodate between 187 and 192 total passengers while United at the moment has absolutely nothing. Some MAX 10s are slated to be delivered in a premium layout while others will have a standard layout. The MAX 10s with a standard layout could potentially accommodate 189-196 total passengers. Combine the existing MAX 10 order with a suspected A321NEO order and United could potentially have 60 A321NEOs and perhaps another 50 or so MAX 10s giving us the critical capacity we need in the 185 -200 seat seat range.
I think the current MAX 10 order remains as is especially if we are only talking about 60 A321NEOs.