The 1840 edition introduced the reading much joy in place of the original such joy. This reading was maintained in the RLDS textual tradition until the third RLDS edition in 1908. The use of much appears to be a typo rather than the result of editing on the part of Joseph Smith for the 1840 edition; apparently the 1840 typesetter accidentally replaced such with much because both words are orthographically similar.
Elsewhere the text has evidence for both such joy and much joy; there is one other example with such and two with much:
The other example of such joy (in Alma 30:35) is extant in the original manuscript.
Summary: Reject the 1840 change in Alma 19:6 from such joy to much joy; the introduction of much into the 1840 edition was probably a simple typo.