Yes you would use the standard syntax.

If your current isp is doing your dns, it also has the reverse entries setup. Who will be doing your dns after you switch? Your new isp? If so it will need to set up the reverse entries.

Keep your eye on the prize. Anyone is the world needs to be able to take your fqdn and get your ip address. And anyone in the world needs to be able to take your ip address and get your fqdn. This means that anyone in the world needs to be able to contact your primary dns server and your secondary dns server. After the switch you will need these two internet visable boxes. Are you clear on that? Do they exist? You worry me with this talk about using a hosts file. Let's take an imaginary example and walk it though....

Now let's say that you're currently using Joe's ISP Service and you plan to switch to Steve's ISP service. And let's say that you have a box called mailguy.something.com with an IP address of 1.2.3.4 and you need to change that to 5.6.7.8.

Joe is running your current nameservers. So his nameservers are authoritative for all of something.com. And his nameserver's are authoritative for 1.2.3.4. So Joe can do both your forward and reverse entries for you.

Now Joe can enter your new forward entry. But Steve is authoritative for 5.6.7.8. Steve would need to do the reverse entry. If Steve does not do DNS, he could delegate the address to your nameservers. Then you could do the reverse. I suppose that Steve could delegate it to Joe too in theory, but that would be an odd thing to do.

But you need to get something.com off of Joe's nameservers. Joe would then continue to handle any reverse entries for Ip addresses that he owns. If you need to continue using Joe's nameservers after you switch to Steve's ISP service, Joe may charge a large fee or something.