This depend how that phone will report supported networks after that change it the link

How this works:
1. Phone tells the Network what frequencies it supports
2. Network makes decision based on frequencies phone can find in that area and then network will give/move phone to specific network 2G/3G/4G and frequency
3. If there is no 1800 available in that area = you will not get it.
4. If there is = you might get it automatically depending if signal strength + congestion level + admission control + couple 10 other dynamically changing parameters in the network are better in 4G than in 3G.

All in all - Network will move you phone automatically to correct place if based on what is available and Network is optimized correctly.