Can we change another dress including under garments after taking ablution?
Can we substitute the voluntary fast like Muharram (Sunnah) for missed fasts during the time of month of Ramadan?
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His Slave and Messenger.
1- A Muslim can change his inner and outer dress after ablution. This act does not nullify his ablution. We do not know any disagreement among scholars concerning this matter. But one should avoid touching his/her private parts during changing of dress since touching private parts is one of the nullifiers of ablution.
2- If a person has to make up his obligatory missed fasting days of Ramadan or any expiation, then it is not permissible for such person to combine the intention of these fasts with the intention of voluntary fast. This matter is known among Muslim jurist by "sharing". The details of this case is as follow:
A- Combining of two means of worship or the combining of two similar things in this matter is permissible. An example for this rule is ceremonial bath. If a person who is Junub (in the state of ritual impurity) takes a bath to remove his impurity and at the same time makes it for Jum'aha prayer then he becomes free from the state of ritual impurity and at the same time he gets the reward mentioned for taking bath for Jum'ah prayer.
B- The requirement of one form of prayer may be satisfied by another form of prayer in some situations. For example, to meet the requirement that a Muslim pray two Rak'ah upon entering a mosque (Tahiyyat ul-Masjid), he may pray two Rak'ah of Sunnah (if it is before Fajr) or Four Rak'ah of Sunnah (if it is before Zuhr). In doing so he will have satisfied the requirement of Tahiyyat ul-Masjid and fulfilled his desire to pray the Sunnah prayer.
C- A specific form of worship which has its own requirements to satisfy its purpose may not be fulfilled by another form of worship. Since these forms of worship must be performed independently, one form does not include the other.
The Muslim scholars have various opinions concerning the rule if one does so. It may considered a voluntary act, or considered making up the missed obligatory acts, or not considered a valid form of worship at all. Each of these three opinions are held by a group of scholars. So we advise our brother that he must intend exclusively the obligatory fast to become free from his obligation. This is desirable and safer than other acts. Know that one does not become free from his obligation until he fulfills it.
Allah Knows best.