This is what's actually happening to a large extent.
While CBTC signaling is being installed, the original fixed-block signaling remains in place. It's nearly impossible to do this otherwise - you'd have to shut the entire line down for the entire time it takes to install new signals - which could be years.
Once the new signal installation is complete the two systems may operate in parallel (with the fixed-block system acting as a possible backup if CBTC fails, and also so older trains can still run).
The issue is that with 24/7 operation, there is no consistent regular time where work like this can be done. It can only be done in short spurts (late-night shutdowns) or the work crews in-lined with train service, which has obvious safety concerns as well as drastically reducing the capacity of the line.
While CBTC signaling is being installed, the original fixed-block signaling remains in place. It's nearly impossible to do this otherwise - you'd have to shut the entire line down for the entire time it takes to install new signals - which could be years.
Once the new signal installation is complete the two systems may operate in parallel (with the fixed-block system acting as a possible backup if CBTC fails, and also so older trains can still run).
The issue is that with 24/7 operation, there is no consistent regular time where work like this can be done. It can only be done in short spurts (late-night shutdowns) or the work crews in-lined with train service, which has obvious safety concerns as well as drastically reducing the capacity of the line.