Hmm, big question. You'd want to get experiance by using Designer to try to do what you are doing in your AcroForms in order to get used to the new UI and field types. The biggest thing will probably be the new scripting model which is quite different. While you can still use some of the Acrobat script objects to do some things, you'll want to convert to the XFA scripting model as much as possible if you want to have success.
You can check out the Designer Dev center here:
http://partners.adobe.com/public/developer/livecycle/designer/devcenter.html . There's a link to some samples you can look at, and also a link to the XML Form Object Model document, which is extremely useful when writing script. As an example, here's what your script would like using the XFA object model. I'm just typing it into this window here, so I can't guarantee it won't have a typo or a little mistake, but should be pretty close.
var dcProceed = true;
// Get the field containers from each page.
for (var nPageCount = 0; nPageCount < xfa.host.numPages; nPageCount++) {
var oFields = xfa.layout.pageContent(nPageCount, "field");
var nNodesLength = oFields.length;
for (var nNodeCount = 0; nNodeCount < nNodesLength; nNodeCount++) {
if (!(this.ui.oneOfChild.className == "button") && (this.mandatory == "error") {
if (this.rawValue == null || this.rawValue == "") {
app.alert("The field with name " + dcLoopField.name + " is required. Please enter a value before submitting.", 1, 0, "Field is required");
dcProceed = false;
break;
}
}
}
}
if (dcProceed) {
var dSubmit = xfa.form.formName.resolveNode("DCSubmitFV1_0");
dSubmit.presence = "hidden";
dSubmit.execEvent("click");
//where dSubmit is a submit type button that points to your URL
}
Also, XFA forms do not use FDF, they use XML, so depending on what you are doing on the server after the submit you may need to change what's going on there too.
Anyways, the first question I would ask if I was you is why you want to switch. If you already have everything you want working and your just switching because it is the "new" way to design forms it's probably not worth it. If your looking to take advantage of functionality it can provide that AcroForms can't, or your looking to use it with the LiveCycle server products then go for it. It's a big change with a definate learning curve, but in the end I think you'll be happy.
Chris
Adobe Enterprise Developer Support