Pro Junior tube swap

[Dan Todd <tuberadio3@xxxxxxxxxx>]

Happy Holidays to one and all!
I  just got a new non-tweed Pro Junior (1X10 @ 15 watts) and want to 
boost the harp volume I can get before feedback.  In the archives, 
knowledgeable folks sometimes recommend switching the 12AX7's for 
12AU7's.  I have never swapped out tubes before.  Is it as easy as: 1)  
unplug amp  2)  remove 12AX7's  3) replace with 12AU7's  4)  plug in 
amp and blow ??
I also plan to play guitar through this amp.  I see so many other 
guitarists/harmonicists on the list these days. I wonder if my guitar 
sound will lose commensurate treble tones (I play in a Robert Junior 
style -- very few ripping post-Hendrix single string high leads -- more 
of a pre-60's Memphis Sonny Boy ensemble sound, if that makes sense).
Any suggestions/ directions or tube brand recommendations would be much 
appreciated -- even a reputable online dealer's URL.
Thanks very much for any help here, and have a great New Year everybody!
Dan Todd