char *func1(char *new,const char *src,const char *repl)
{
    size_t src_len,repl_len;

    if(src && repl)
    {
        src_len = strlen(src);
        repl_len = strlen(repl);
        if(src_len >= repl_len)
        {
            new[src_len] = 0;
            while(repl_len)
            {
                new[--src_len] = repl[--repl_len];
            }
            if(src_len)
            {
                while(--src_len)
                {
                    new[src_len] = src[src_len];
                }
            }
        }
    }
    return new;
}

char *func2(char *new,const char *repl,size_t nchars)
{
    //last nchars only (inluding the nul char)
    size_t src_len,repl_len;

    if(new &&src && repl)
    {
        new[--nchars] = 0;
        src_len = strlen(src);
        repl_len = strlen(repl);
        if(src_len >= repl_len)
        {
            while(repl_len && nchars)
            {
                new[--nchars] = repl[--repl_len];
                --src_len;
            }
            if(src_len && nchars)
            {
                while(--src_len && --nchars)
                {
                    new[nchars] = src[src_len];
                }
            }
        }
    }
    return new;
}

int main()
{
    char *string =  "S2_2018_08_09_B02_10m.tif";
    char *repl =  "SCL_10m.tif";
    char new[256];
    printf("func1 - new = \"%s\",src = \"%s\",repl = \"%s\"\n",func1(new,string,repl),repl);
    printf("func2 - new = \"%s\",func2(new,repl,15),repl); 
    printf("func2 - new = \"%s\","123456789_SCL_10m.tif",repl); 
    return 0;
}