A template is much more than just a clean file from a previous project. A template is a collection of best practices, aspirations, and starting points to save you time. This blog post covers a lot of my feelings on templates:
http://www.shoegnome.com/2015/12/09/bim ... ting-firm/
(and from there you can find many, many more). As to when to start a new template:
In an ideal world you'd start from scratch every version. That's not always feasible though. So I agree with Eduardo. Major changes in ARCHICAD require a rebuilding (AC16 to AC17 for BMats, for instance). Other versions don't require as much recreation (AC20 to AC21). However there are other lesser reasons to rebuild from scratch. If GS redoes how they do Attributes, as they do secretly every few versions, that's a reason to rebuild from scratch and align your template (where valuable) with what comes OOTB.
Another reason to rebuild from scratch is to fix bad habits. If you look back at the various versions of my Open Template (http://www.shoegnome.com/tag/shoegnome-open-template/
) you can see where and why I felt it was necessary to rebuild completely. Sometimes it was to fix Pens and Pen Sets, other times it was to align Attributes with what GSNA does or to make my Layers work better with Graphic Overrides. We work the way we do because we have brilliant ideas for how to use ARCHICAD. But ARCHICAD changes over time, so our brilliant ideas become garbage or at least archaic.
If you have been migrating your template from version to version for many years, it's probably time to tear it apart and rebuild it. Bring forward what works and don't be afraid to ditch what doesn't. It's time to rebuild not because of anything special about 21, but because it's probably best to not let the core of a template get more than a few versions old. If you have the time completely reconstruct everything in a clean AC21 file. If you don't want to do that, judiciously use the Attribute Manager to migrate Attributes and carefully copy and paste other things. Whenever you can though rebuild manually in the new file to prevent errors, to avoid migrated garbage, and also review what you do.
Okay, so that's all a long winded way of saying templates are about workflow. If your workflow takes proper advantage of all the recent features of ARCHICAD, it's probably fine to just migrate another year forward. If your workflows don't play well or fail to use things like renovation filters, Graphic Overrides, the new Favorites, CineRender, ARCHICAD Properties, etc, it's time to return to the fundamentals and cause a little pain as you rework how you work.
BTW, the same goes with keyboard shortcuts:
http://www.shoegnome.com/2017/06/28/sho ... chicad-21/
(edit: I agree 100% with the first half of what James wrote. I respectfully and lovingly disagree with his second half. Sometimes a break from the past is necessary—for those of us who didn't start their templates perfectly the first time like James did).
Jared Banks, AIA
ArchiCAD blog: www.shoegnome.com
ArchiCAD tutorial videos: www.youtube.com/user/ShoegnomeLLC
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