You can make the conversion, but it might be more fun to just buy a CS-15/17 radio.  The filters are different and it is difficult to remove the DIY7-7 coils and replace them with DIY7-21 as well as the associated capacitors.
One thing that you might consider is my dual band option for a KN-Q7A or a CS-series radio.  It comes with the filters needed.  However, I have not checked out the 15 or 17 Meter filters, yet.  I will be doing that this week or next week, I hope.
But you can add the Sandwich to your radio now and get that part of the conversion working.  The Sandwich will give you full band coverage instead of partial band coverage.  You can order that from or from me in the US.
My add: visit

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