Python 3 solution inspired by/adapting the simple C++ solution a few comments down:

    # Write your code here
diff = [0]*n
temp = k
il = [int(x) for x in s]

#make a base palindrome first if possible
for i in range(len(il)//2 +1):
    if il[i] != il[len(il)-1-i] and temp == 0:
        return '-1'
    if il[i] != il[len(il)-1-i]:
        cmax = max( il[i], il[len(il)-1-i])
        il[i] = cmax
        il[len(il)-1-i] = cmax
        temp -= 1
        diff[i] = 1


#make all as many elements 9s as possible starting from the biggest digit
for i in range(len(il)//2 +1):

    #if no more changes can be made return the completed string
    if temp == 0:
        return ''.join(map(str, il))

    if diff[i] == 1 or i == len(il)-1-i:
        if il[i] != 9:
            il[i] = 9
            il[len(il)-1-i] = 9
            temp -= 1
    elif diff[i] == 0 and temp >= 2:
        if il[i] != 9:
            il[i] = 9
            il[len(il)-1-i] = 9
            temp -= 2

return ''.join(map(str, il))