Hi -this is Sue from GE. Thanks for your interest in our GE model JVM3160RFSS. We appreciate your loyalty!
Our 30" over-the-range microwaves are designed to be installed in the same standard upper cabinet space (30" min. width, 12" min.-13" max. depth). The height and depth of the models may vary slightly. The top mounting is basically the same. You will need to use the back wall mounting bracket included with the new model as they are similar but not interchangeable.
Your model, JNM1851SM2SS was shipped assembled for recirculating (indoor) venting. Model JVM3160RFSS is shipped assembled for outdoor venting. If you choose model JVM3160, you will also need to purchase the optional recirculating charcoal filter JX81L and follow the recirculating venting instructions in the installation guide.
You may want to consider model JNM3161RFSS. It is the same model except is shipped assembled for recirculating (indoor) venting. You can view online at:
Thanks for checking with us. Enjoy!