#include <iostream>
#include <cstring>
 
char* str_replace(char* str, size_t size, const char* old, const char* ns){
    size_t len, num;
    char*  tmp  = str;
    size_t len1 = strlen(old);
    size_t len2 = strlen(ns);
 
    if(len2 > len1){
        len = strlen(str);
        num = len2 - len1;
    }
 
    while((str = strstr(str, old)) != NULL){
        if(len1 > len2){
            strncpy(str, ns, len2);
            strcpy(str + len2, str + len1);
        } else if(len1 < len2){
 
            if((len + num + 1) >= size)
                break;
 
            memmove(str + num, str, (len - (size_t)(str - tmp)) * sizeof(char));
            strncpy(str, ns, len2);
            len += num;
        } else
            strncpy(str, ns, len2);
        str += len2;
    }
    return tmp;
}
 
int main(void){
    char str[256] = "дуб, (дуб), липа, дуб, {дуб}... дуб";
    char old[]    = "дуб";
    char ns[]     = "берёза";
 
    std::cout << str << std::endl;
    std::cout << str_replace(str, sizeof(str)-1, old, ns) << std::endl;
    return 0;
}

#include <algorithm>
#include <iostream>
#include <iterator>
#include <sstream>
#include <string>
 
int main()
{
    const std::string str = "mother father sosage mother you", from = "mother", to = "grandmother";
    std::istringstream ist(str);
    std::replace_copy(std::istream_iterator<std::string>(ist), std::istream_iterator<std::string>(),
        std::ostream_iterator<std::string>(std::cout, " "), from, to);
}

Text1.Text = Replace(Text1.Text, "Петр", "Иван")