InfoPath Dev

Hi All, I am working on an unfortunately very messy Info path form at work and there is a desire to change one of the controls from a drop down box (which only allows you to select 1 user) to a Multiple selections list box (to enable selecting multiple). This particular control has not been named (is basically called "field 37" - the system allocated name when creating the control). If I created a new multiple selections list box and named it "field 37" , deleting the current "field 37" drop down box - could this work? i.e. If the current drop down list has "John Wayne" and "Bruce Wayne" in the list, would they carry over to the new multiple selections list box of the same name "field 37" ? Apologies if I am not being clear, hope it makes sense.

 Here's what I'd do:

1. In the data source task pane, right click field37 and select Properties
2. Set the field to repeating (see screenshot)
3. Remove the dropdown control from the form
4. Drag field37 onto the form and select multiple select list box

As long as the options previously selected are still in the available options, you shouldn't lose data (I didn't in my testing). However, to be safe:

1. Download one of the existing filled out forms from SharePoint
2. Under File > Advanced Options in your InfoPath template, while in design mode, find the "preview" category
3. There is an option to "preview with existing data" - browse to your downloaded file

Preview the form and make sure the previously selected option is still showing. More on this technique here: https://www.infopathdev.com/blogs/hilary/archive/2009/03/13/first-do-no-harm.aspx 

Thanks Hilary for the reply, I tried changing field 37 to repeatable but it's a drop-down box and does not have that option (tried to insert pic here but it just seems to add as an attachment). I've followed your guide in the link (and commentary here), what I did was; - Downloaded a sample form from the SharePoint library - Connected this new template as advised above, previewed and works beautifully (this is great, didn't know you could do this, thank you) - Went into design mode and deleted field37 - Added in a replacement multi select list box and renamed it to field 37 and moved to appropriate group. - Previewed the form again and it is working as intended. Would this signify that it would work also once published? Appreciate your response - thank you again !

So - the approach I took was just to remove the control and then change the field - my guess is that with the new field having the same name as the old, you will likely be ok. I would be careful tho - create a test form in the library, save a backup of the currently published form. Publish your new form at a time when no one is using the library. Then open your test form with the published form and make sure everything is as anticipated. Sometimes Browser forms are more finicky than Filler and when we test in Preview we are testing in Filler. Does that make sense?

 It makes perfect sense and the majority of users will be completing via browser which is what scares me a little. I will go with the test form in the library and see how that goes over the weekend when no one is using it, thank you Hilary.

I'd just like to say that over the past few months I've really brushed  up on my skills with InfoPath and SharePoint following all your content - avid fan and really appreciate your knowledge !

I'm glad I've been able to help you :)