Hello B.Bouma,
You can change template folder by performing the following steps :
1. Copy Template Folder
2. Pate Template Folder
3. Folder after copy
4. Rename folder after copy
5. Folder after Rename
6. Inside the new folder :
a) File en-GB.tpl_business_plan_ii.ini :
en-GB.tpl_business_plan_ii.ini
=>
en-GB.tpl_your_template.ini
AND
b) File templateDetails.xml : You open file and change codes :
<name>Business_Plan_II</name>
=> changed :
<name>Your_Template</name>
<language tag="en-GB">en-GB.tpl_business_plan_ii.ini</language>
=> changed :
<language tag="en-GB">en-GB.tpl_your_template.ini</language>
<file>en-GB.tpl_business_plan_ii.ini</file>
=> changed :
<file>en-GB.tpl_your_template.ini</file>
Then , you need copy file en-GB.tpl_your_template.ini into folder language of website :
7. Login in administrator :
8. Discover :
9. Install Your Template
10. DONE : Now you can edit on your new template folder.

Happy new year