SMD vs. THT Difference
SMD pros: smaller size, can be assembled by pick-and-place machine, no lead inductance or smaller inductance for packages containing leads.
Through-Hole pros: bigger package - better power dissipation (e.g. resistors). Although it depends on PCB design. If SMD component is mounted on copper plane it may have better thermal drain as mentioned in comments below.
Regarding cost - SMD mostly cheaper.
Replacing SMD with through-hole component is not always possible. For example in RF circuits, where lead inductance plays significant role.