There's a significant difference in terms of region and item layout between themes 24 and 25. Theme 24 uses legacy table-based layout, whilst 25 uses CSS grid layout. Exactly which theme are you using?
Which page template are you using?
I have a simple requirement, On one page there is a interactive report which when user selects one row opens a page which shows that record in a form layout, where user can update that record. Now most of the fields are read only but one or two could be updated. However, When I select the standard form template,
What do you mean by "standard form template"?
even though we have the option of playing a bit with "New row" setting for an item, We need to slightly rearrange the items a bit more, some set of display items will be together, some might have a couple of lines of gap, some might be side by side etc.
You'll have to be specific in terms of the layout. Please create examples on apex.oracle.com, and provide some kind of illustration of any layout you are unable to achieve.
Was wondering what is the best way to modify the form layout without having to do a custom layout in HTML_DB etc.
What do you mean by "custom layout in HTML_DB"? "HTML DB" is the former name for APEX, so doing a custom layout in HTML DB equates to doing a custom layout in APEX—which is what you're asking about.