[Koha] changes to marc structure and authorities

[Georgia Katsarou]

Hello!  I am an new Koha user and I have some questions just to be sure that I won't destroy anything. 
We have passed nearly 4000 titles to Koha but the marc structure is not right and we have not made the settings for the authorities from the beginning.  If we make all the necessary changes and fix the authorities now we are going to cause any damage to the records that already exist in the system?
Moreover I was wondering if we can somehow change the interface of the OPAC or find more themes.  We are an academic library and sometimes we have 65 copies of the same textbook.  When we make a search on the certain title at the OPAC, at the "Available at" column it shows the location of each copy making huge lists since we talk about 65 copies.  We do not have any branches and we named our library "Main" so there is no reason for the opac to show that we have 65 copies in the "Main".

Thank you very much

Georgia Katsarou
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