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 the scholars concerning this matter. But one should avoid touching his/her private parts when changing his/her dress since touching the private parts is one of the nullifiers of ablution.

2- If a person has to make up for his obligatory fasting that he missed during Ramadan or any expiation, then it is not permissible for such a person to combine the intention of these fasts with the intention of voluntary fast. This matter is known among the Muslim jurist by "sharing". The details of this case are as follows:

A- Combining two means of worship or two similar things in this matter is permissible. An example for this ruling is Ghusl (ritual bath). If a person who is in the state of ritual impurity takes a bath in order to remove his impurity and at the same time intends it for Friday 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 Friday 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 of praying two Rak'ahs upon entering a mosque (Tahiyyatul-Masjid, which is the prayer of salutation to the mosque), he may pray two Rak'ahs of Sunnah (if it is before Fajr) or Four Rak'ahs of Sunnah (if it is before Zuhr). In doing so, he meets the requirement of praying Tahiyyatul Masjid and meets the requirement of praying 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 ruling if one does so. It may be 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 is 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 and in order to be on the safe side. You should know that one does not become free from his obligation until he fulfills it.

Allah knows best.